README.md: Update build instructions and add build flow diagram

README.md

@@ -1,33 +1,182 @@
 # The System Package Data Exchange (SPDX®) Specification
 
-The System Package Data Exchange (SPDX®) specification is an open standard capable of representing systems with software components in as SBOMs (Software Bill of Materials) and other AI, data and security references supporting a range of risk management use cases.
+The System Package Data Exchange (SPDX®) specification is an open standard
+capable of representing systems with software components in as SBOMs
+(Software Bill of Materials) and other AI, data and security references
+supporting a range of risk management use cases.
 
-The SPDX standard helps facilitate compliance with free and open source software licenses by standardizing the way license information is shared across the software supply chain. SPDX reduces redundant work by providing a common format for companies and communities to share important data about software licenses and copyrights, thereby streamlining and improving compliance.
+The SPDX standard helps facilitate compliance with free and open source
+software licenses by standardizing the way license information is shared across
+the software supply chain. SPDX reduces redundant work by providing a common
+format for companies and communities to share important data about software
+licenses and copyrights, thereby streamlining and improving compliance.
 
 This repository holds under active development version of the specification as:
 
-* [MarkDown](https://github.com/spdx/spdx-spec/tree/master/chapters) (`master` branch)
-* HTML (gh-pages branch, built on every commit to `master` and `development/` branches)
-  * [Current](https://spdx.github.io/spdx-spec/v3.0/)
+- [Markdown](https://github.com/spdx/spdx-spec/tree/development/v3.0.1/docs)
+  (`development/v3.0.1` branch)
+- HTML (`gh-pages` branch, built on every commit to `master` and `development`
+  branches)
+  - Current: <https://spdx.github.io/spdx-spec/v3.0/>
 
-See for the official [releases of the specification](https://spdx.org/specifications) or additional information also the [SPDX website](https://spdx.org).
+The model itself is under active development at
+[spdx/spdx-3-model](https://github.com/spdx/spdx-3-model/)
+repo (`main` branch).
 
-## Specification Structure
+See for the official
+[releases of the specification](https://spdx.org/specifications)
+or additional information also the [SPDX website](https://spdx.org).
 
-The specification consists of a model which is generated from the [spdx-3-model](https://github.com/spdx/spdx-3-model) repository and additional information in the `docs` directory.
+## Specification structure
 
-The `examples` directory contains examples of various SPDX serializations for the current version of the spec.
+This repository consists of these files and directories:
 
-# Building the specification
+- `bin/` - Scripts for spec generation.
+- `docs/` - Specification content:
+  - `annexes/` - Annexes for the specification.
+  - `css/` - Style sheets for HTML.
+  - `images/` - Model diagrams. These image files are to be generated from a
+    diagram description file
+    [model.drawio](https://github.com/spdx/spdx-3-model/blob/main/model.drawio)
+    in `spdx/spdx-3-model` repo and manually copied here.
+  - `licenses/` - Licenses that used by the SPDX specifications.
+  - `model/` - Model files*. This subdirectory _is to be created_ by a script
+    from `spdx/spec-parser` repo, using model information from
+    `spdx/spdx-3-model` repo (see the build instructions below).
+- `examples/` - Examples of various SPDX serializations for the current version
+  of the spec.
+- `mkdocs.yml` - MkDocs recipe for the spec documentation generation. The
+  inclusion of model files and the order of chapters are defined here.
 
-## Prerequisites
+The specification consists of documents in the `docs/` directory from this
+`spdx/spdx-spec` repository and a model which is generated from Markdown files
+in the `spdx/spdx-3-model` repository.
 
-You have to [MkDocs](http://mkdocs.org) installed on your machine. If you don't have it yet installed please follow these [installation instructions](http://www.mkdocs.org/#installation).
+Note: The model Markdown files in the `spdx/spdx-3-model` repository use a
+constrained format. Only a limited set of headings are allowed for processing
+by the spec-parser.
 
-## Building HTML
+## Building the specification
 
-    # Execute built-in dev-server that lets you preview the specification
-    $ mkdocs serve
+The specification building flow looks like this:
 
-    # Building static HTML site
-    $ mkdocs build
+```text
+  +-------------------+
+  |[spdx-3-model]     |
+  | |                 |
+  | +- model/        ---- Constrained-Markdown files -+
+  | |                 |                               |
+  | +- model.drawio  -----------------+               |
+  +-------------------+               |               |
+                                      |               |
+                                      |               |
+  +-------------------+               v               |
+  |[spdx-spec]        |            draw.io            |
+  | |                 |            (manual)           |
+  | +- docs/          |               |               |
+  |    |              |               |               |
+  |    +- annexes/    |               |               v
+  |    |              |               |         spec-parser
+  |    +- images/  <---- PNG images --+               |
+  |    |              |                               |
+  |    +- licenses/   |                               |
+  |    |              |                               |
+  |    +- model/   <----- Processed Markdown files ---+
+  |    |              |
+  |    +- index.md    |
+  |    |              |
+  |    +- *.md        |
+  +-------------------+
+          |
+    mike & mkdocs
+          |
+          v
+  +-------------------+
+  |   HTML website    |
+  +-------------------+
+```
+
+### Prerequisites
+
+Apart from Git and Python, you have to have [MkDocs](http://mkdocs.org)
+installed on your machine. If you don't have it yet installed please follow
+these [installation instructions](http://www.mkdocs.org/#installation).
+
+[WeasyPrint](https://doc.courtbouillon.org/weasyprint/stable/first_steps.html#installation)
+is also required for generating PDF files. To disable PDF generation, comment
+out the these lines in your `mkdocs.yml` configuration file:
+
+```yaml
+#- pdf-export:
+#    combined: true
+```
+
+### Preparing input files
+
+Next, you have to prepare the model files, the other specification files,
+and the model parser, by cloning these repositoriess:
+[`spdx/spdx-3-model`](https://github.com/spdx/spdx-3-model),
+[`spdx/spdx-spec`](https://github.com/spdx/spdx-spec), and
+[`spdx/spec-parser`](https://github.com/spdx/spec-parser)
+to these paths: `spdx-3-model`, `spdx-spec`, and `spec-parser`, respectively:
+
+```shell
+git clone https://github.com/spdx/spdx-3-model.git
+git clone https://github.com/spdx/spdx-spec.git
+git clone https://github.com/spdx/spec-parser.git
+```
+
+Install prerequisites for Python:
+
+```shell
+pip3 install -r spdx-spec/requirements.txt
+pip3 install -r spec-parser/requirements.txt
+```
+
+### Generating intermediate Markdown files for MkDocs
+
+Validate model files and generate intermediate files to be used by the HTML
+builder (MkDocs):
+
+```shell
+python3 spec-parser/main.py spdx-3-model/model spdx-spec/docs/model
+```
+
+Now you will have your well-formatted model files ready in
+`spdx-spec/docs/model` directory.
+
+Inside this directory there will be two components:
+
+- Model ontology and diagram files (`model.plantuml`, `spdx-context.jsonld`,
+  `spdx-model.dot`, `spdx-model.json-ld`, `spdx-model.pretty-xml`,
+  `spdx-model.ttl`, `spdx-model.xml`) -- these files are ready to be consumed
+  directly.
+- Formatted Makdown files (`*.md`) in a number of subdirectories -- these
+  files are to be consumed by MkDocs in the next step.
+
+### Building HTML
+
+Once all the spec and model files are ready, we will use MkDocs to put them
+together as a website.
+
+In side `spdx-spec/` directory, execute a built-in dev-server that let you
+preview the specification:
+
+```shell
+mkdocs serve
+```
+
+Or building a static HTML site:
+
+```shell
+mkdocs build
+```
+
+## Configuring the website
+
+Inside `spdx-spec/` directory, there is a file `mkdocs.yml`. This is a
+configuration file for MkDocs.
+
+Files intended for display and linking in the navigation bar should be
+included in the `nav:` section. The order of filenames in this section
+determines their order on the navigation bar.