Some additions to the CONTRIBUTING guide (#1685)

* Some additions to the CONTRIBUTING guide

- how to run black if so desired
- install instructions for devs (following GH action)
- running tests
- minor fixups

If there is a guide on how to best add new models, it would be a good
idea to link it here, since I imagine this is what many contributors
would need most help with.

* [skip ci] empty commit to skip ci

CONTRIBUTING.md

@@ -4,9 +4,9 @@ Contriubtions to `timm` for code, documentation, tests are more than welcome!
 
 There haven't been any formal guidelines to date so please bear with me, and feel free to add to this guide.
 
-# Code
+# Coding style
 
-Code linting and auto-format (black) are not currently in place but open to consideration. In the meantime, the style to follow is (mostly) aligned with Google's guide: https://google.github.io/styleguide/pyguide.html
+Code linting and auto-format (black) are not currently in place but open to consideration. In the meantime, the style to follow is (mostly) aligned with Google's guide: https://google.github.io/styleguide/pyguide.html. 
 
 A few specific differences from Google style (or black)
 1. Line length is 120 char. Going over is okay in some cases (e.g. I prefer not to break URL across lines).
@@ -51,8 +51,15 @@ This is YES:
 
 When there is descrepancy in a given source file (there are many origins for various bits of code and not all have been updated to what I consider current goal), please follow the style in a given file.
 
-PR with pure formatting / style fixes will be accepted but only in isolation from functional changes, best to ask before starting such a change.
+In general, if you add new code, formatting it with black using the following options should result in a style that is compatible with the rest of the code base:
 
+```
+black --skip-string-normalization --line-length 120 <path-to-file>
+```
+
+Avoid formatting code that is unrelated to your PR though.
+
+PR with pure formatting / style fixes will be accepted but only in isolation from functional changes, best to ask before starting such a change.
 
 # Documentation
 
@@ -64,8 +71,42 @@ When type annotations are used for a function, as per the Google pyguide, they s
 
 There are a LOT of gaps in current documentation relative to the functionality in timm, please, document away!
 
+# Installation
+
+Create a Python virtual environment using Python 3.10. Inside the environment, install the following test dependencies:
+
+```
+python -m pip install pytest pytest-timeout pytest-xdist pytest-forked expecttest
+```
+
+Install `torch` and `torchvision` using the instructions matching your system as listed on the [PyTorch website](https://pytorch.org/).
+
+Then install the remaining dependencies:
+
+```
+python -m pip install -r requirements.txt
+python -m pip install --no-cache-dir git+https://github.com/mapillary/inplace_abn.git
+python -m pip install -e .
+```
+
+## Unit tests
+
+Run the tests using:
+
+```
+pytest tests/
+```
+
+Since the whole test suite takes a lot of time to run locally (a few hours), you may want to select a subset of tests relating to the changes you made by using the `-k` option of [`pytest`](https://docs.pytest.org/en/7.1.x/example/markers.html#using-k-expr-to-select-tests-based-on-their-name). Moreover, running tests in parallel (in this example 4 processes) with the `-n` option may help:
+
+```
+pytest -k "substring-to-match" -n 4 tests/
+```
+
+## Building documentation
+
+Please refer to [this document](https://github.com/huggingface/pytorch-image-models/tree/main/hfdocs).
+
 # Questions
 
-If you have any questions about contribution, where / how to contribute, please ask in the Discussions (there is a `Contributing` topic).
-
-
+If you have any questions about contribution, where / how to contribute, please ask in the [Discussions](https://github.com/huggingface/pytorch-image-models/discussions/categories/contributing) (there is a `Contributing` topic).