Update contributing

CONTRIBUTING.md

@@ -4,9 +4,7 @@ Contributions - in the form of code, bugs, or ideas - are very welcome!
 
 ## Reporting bugs and suggesting enhancements
 
-The [issues list](https://github.com/mnot/redbot/issues) is the best place to report
-problems or suggest new features. Before you submit something, it would be great if you had a look
-through the current issues to see if it's already there.
+The [issues list](https://github.com/mnot/redbot/issues) is the best place to report problems or suggest new features. Before you submit something, it would be great if you had a look through the current issues to see if it's already there.
 
 
 ## Making code contributions
@@ -18,32 +16,23 @@ A quick overview of the repository contents:
 * `bin/` has the command-line and Web (CGI and standalone daemon) executables that are the glue between the outside world and REDbot
 * `redbot/` contains the redbot Python module:
   * `assets/` contains static assets for the Web UI
-  * `formatter/` holds the different output formatters (e.g., HTML, plaintext, [HAR](http://www.softwareishard.com/blog/har-12-spec/)) for the check results
-  * `message/` checks one message, either request or response (but currently, mostly responses)
-    * `headers/` checks individual HTTP headers
+  * `formatter/` holds the different output formatters (e.g., HTML, plaintext, [HAR](http://www.softwareishard.com/blog/har-12-spec/)) for presenting the check results
   * `resource/` defines the actual checker for a given URL, as `HTTPResource` in `__init__.py`. Lower-level fetching of resources is handled in `fetch.py`.
-    * `active_check/` doing a check on a resource might involve making additional requests, e.g., for ETag validation. Those checks are defined here
-  * `syntax/` has a collection of [ABNF](https://tools.ietf.org/html/rfc5234) translated into regex, for checking syntax
+    * `active_check/` Additional checks on a resource that involve making additional requests, e.g., for ETag validation
   * `webui/` contains the engine for Web-based interaction with REDbot
 * `src/` contains the source files for the JavaScript and CSS files
 * `test/` guess what's here?
 
-Generally, running a check involves instantiating a new `HTTPResource` object, which is a subclass
-of `RedFetcher`. It's in charge of making all of the HTTP requests necessary to test that resource,
-and feeds the results into `HttpMessage` objects that then checks its various aspects, especially
-headers. If `descend` is true, the response will be parsed for links and `HTTPResource`s will be
-created for each of them. When it's all done, the "root" `HTTPResource` will be fed to a
-`Formatter` for presentation.
+Most message-level checks are performed by the [httplint](https://pypi.org/project/httplint/) library; REDbot makes requests, feeds the responses into httplint, and presents the results. Additionally, REDbot makes "active" checks against resource behaviour for things like content negotiation and validation.
 
-If you plan to support a new HTTP header in REDbot (a common task), see the [guide for Header
-Development](https://github.com/mnot/redbot/blob/master/redbot/message/headers/README.md).
+Checking a resource involves instantiating a new `HTTPResource` object, which is a subclass of `RedFetcher`. It's in charge of making all of the HTTP requests necessary to test that resource, and feeds the results into httplint's `HttpMessageLinter` objects that then checks various aspects of the messages, especially headers. If `descend` is true, the response will be parsed for links and `HTTPResource`s will be created for each of them. When it's all done, the "root" `HTTPResource` will be fed to a `Formatter` for presentation.
 
 
 ### Coding conventions
 
 We use [black](https://pypi.org/project/black/) for Python formatting, and [standard](https://standardjs.com) for JavaScript; both can be run with `make tidy`.
 
-All Python functions and methods need to have type annotations. See `pyproject.toml` for specific pylint and mypy settings.
+All Python functions and methods need to have type annotations. See `pyproject.toml` for specific pylint and mypy settings. `make typecheck` and `make lint` are the relevant make targets.
 
 
 ### Setting up a development environment