[DOCS] Contribution docs page

[Kludex]

Similar to https://www.python-httpx.org/contributing/ but addapted to uvicorn.

[LuluNwenyi]

Hi @Kludex I'd like to work on this but could you elaborate so I could get more context

[Kludex]

Hi @LuluNwenyi ! Thanks for the interest!

There are some things on the httpx page that doesn't make sense for us, like the Development proxy setup section.
And on the Releasing section, we don't have a __version__.py, but we change the version on the __init__.py.

Besides, on the Reporting Bugs or Other Issues section, on httpx is recommended to open a discussion, and there are some labels, but we don't have here, so I guess on uvicorn the idea would be to first discuss on our Gitter server, and if it makes sense, create an issue here.

The page should be called contributing.md, and it should be the last on the docs.

[LuluNwenyi]

Alright. I can be assigned to this.'d love to give it a go

[Kludex]

Go ahead!