Update the shape description

Adapted from #52 by @esgomezm

README.md

@@ -12,7 +12,7 @@ To get a quick overview of the config file, see an example file [here](https://g
 
 ## Model Specification
 
-A model entry in the bioimage.io model zoo is defined by a configuration file `<model name>.model.yaml`.
+A model entry in the bioimage.io model zoo is defined by a configuration file `model.yaml`.
 The configuration file must contain the following \[optional\]* keys (* denotes that the field is optional depending on input to another field):
 
 
@@ -36,7 +36,7 @@ The authors are the creators of the specifications and the primary points of con
 
 - `cite`
 A citation entry or list of citation entries.
-Each entry contains of a mandatory `text` field and either one or both of `doi` and `url`.
+Each entry contains a mandatory `text` field and either one or both of `doi` and `url`.
 E.g. the citation for the model architecture and/or the training data used.
 
 - `git_repo`
@@ -62,12 +62,20 @@ Must be a list of *tensor specification keys*.
   *tensor specification keys*:
   - `name` tensor name
   - `data_type` data type (e.g. float32)
-  - `data_range` tuple of (minimum, maximum)
-  - `axes` string of axes identifying characters from: btczyx
-  - `shape` specification of tensor shape\
-    Either as *exact shape with same length as `axes`*,\
-    or (only for input) as {`min` *minimum shape with same length as `axes`*, `step` *minimum shape change with same length as `axes`*},\
-    or (only for output) as {`reference_input` *input tensor name*, `scale` *list of factors 'output_pix/input_pix' for each dimension*, `offset` *position of origin wrt to input*, `halo` *the halo to crop from the output tensor (for example to crop away boundary efects or for tiling)*}
+  - `data_range` tuple of (minimum, maximum), e.g. [0, 1]
+  - `axes` string of axes identifying characters from: btczyx. Same length and order as the axes in `shape`.
+  - `shape` specification of tensor shape. It can be specified in three diffeent ways:
+    1. as exact shape with same length as `axes`, e.g. `shape: [1, 512, 512, 1]`
+    2. (only for input) as a sequence of valid shapes.\
+       The valid shapes are given by `shape = min + k * step for k in {0, 1, ...}`. Specified by the following fields: 
+       - `min` the minimum input shape with same length as `axes`
+       - `step` the minimum shape change with same length as `axes`
+    3. (only for output) in reference to the shape of an input tensor.\
+       The shape of the output tensor is `shape = shape(input_tensor) * scale + offset`. Specified by the following fields:
+       - `reference_input` name of the reference input tensor
+       - `scale` list of factors 'output_pix/input_pix' for each dimension
+       - `offset` position of origin wrt to input 
+       - `halo` the halo to crop from the output tensor (for example to crop away boundary efects or for tiling). The halo should be croped from both size, i.e. `shape_after_crop = shape - 2 * halo`.
   - `[preprocessing]` (only for input) list of transformations describing how this input should be preprocessed. Each entry has these keys:
     - `name` name of preprocessing (see [supported_formats_and_operations.md#preprocessing](https://github.com/bioimage-io/configuration/blob/master/supported_formats_and_operations.md#preprocessing) for valid names).
     - `[kwargs]` key word arguments for `preprocessing`.
@@ -134,19 +142,19 @@ For example:
 - `[sample_inputs]` list of URIs to sample inputs to illustrate possible inputs for the model, for example stored as png or tif images.
 - `[sample_outputs]` list of URIs to sample outputs corresponding to the `sample_inputs`.
 
-- `weights` The weights for this model. Weights can be given for different formats, but should otherwise be equivalent.
+- `weights` The weights for this model. Weights can be given for different formats, but should otherwise be equivalent. The available weight formats determine which consumers can use this model.
    - `weight_format` Format of this set of weights. Weight formats can define additional (optional or required) fields. See [supported_formats_and_operations.md#Weight Format](https://github.com/bioimage-io/configuration/blob/master/supported_formats_and_operations.md#weight_format)
         - `[authors]` a list of authors.
         - `source` link to the weights file. Preferably an url to the weights file.
-        - `sha256` SHA256 checksum of the model weight file specified by `source` (see `models` section above for how to generate SHA256 checksum)
+        - `sha256` SHA256 checksum of the model weight file specified by `source` (see `sha256` section for how to generate the checksum)
         - `[attachments]` weight specific attachments that will be included when generating the model package.
 
 - `[run_mode]` Custom run mode for this model: for more complex prediction procedures like test time data augmentation that currently cannot be expressed in the specification. The different run modes should be listed in [supported_formats_and_operations.md#Run Modes](https://github.com/bioimage-io/configuration/blob/master/supported_formats_and_operations.md#run modes)
   - `name` the name of the `run_mode`
   - `[kwargs]` keyword arguments for this `run_mode`
 
 - `[config]`
-A custom configuration field that can contain any other keys which are not defined above. It can be very specifc to a framework or specific tool. To avoid conflicted defintions, it is recommended to wrap configuration into a sub-field named with the specific framework or tool name. 
+A custom configuration field that can contain any other keys which are not defined above. It can be very specifc to a framework or specific tool. To avoid conflicted definitions, it is recommended to wrap configuration into a sub-field named with the specific framework or tool name. 
 
 For example:
 ```yaml