Create CONTRIBUTING.md

Author: rwightman

CONTRIBUTING.md

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+*This guideline is very much a work-in-progress.*
+
+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
+
+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).
+2. Hanging indents are always prefered, please avoid aligning arguments with closing brackets or braces.
+
+Example, from Google guide, but this is a NO here:
+```
+   # Aligned with opening delimiter.
+   foo = long_function_name(var_one, var_two,
+                            var_three, var_four)
+   meal = (spam,
+           beans)
+
+   # Aligned with opening delimiter in a dictionary.
+   foo = {
+       'long_dictionary_key': value1 +
+                              value2,
+       ...
+   }
+```
+This is YES:
+
+```
+   # 4-space hanging indent; nothing on first line,
+   # closing parenthesis on a new line.
+   foo = long_function_name(
+       var_one, var_two, var_three,
+       var_four
+   )
+   meal = (
+       spam,
+       beans,
+   )
+
+   # 4-space hanging indent in a dictionary.
+   foo = {
+       'long_dictionary_key':
+           long_dictionary_value,
+       ...
+   }
+```
+
+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.
+
+
+# Documentation
+
+As with code style, docstrings style based on the Google guide: guide: https://google.github.io/styleguide/pyguide.html
+
+The goal for the code is to eventually move to have all major functions and `__init__` methods use PEP484 type annotations.
+
+When type annotations are used for a function, as per the Google pyguide, they should **NOT** be duplicated in the docstrings, please leave annotations as the one source of truth re typing.
+
+There are a LOT of gaps in current documentation relative to the functionality in timm, please, document away!
+
+# Questions
+
+If you have any questions about contribution, where / how to contribute, please ask in the Discussions (there is a `Contributing` topic).
+
+