Create CONTRIBUTING.md

[clairecharles]

Please review the following checklist.

Fixes #2933

• Docstrings on all new or modified functions / classes
• Updated documentation
• Updated CHANGELOG.md (where appropriate)

[rodrigogiraoserrao]

Hey Claire, thanks for your PR.

While I think you nailed the welcoming tone for the document, I think the content is not what we are going for.
I think we can drop most of the generic advice, for example the style guide for commit messages or how to create a pull request.
Instead, what we can do is link to proper guides/documentation when needed.

A great alternative, that applies to pretty much everything else, is that we can point to examples of things we've done and that contributors can copy/mimic.

So, instead of focusing on these generic things that are more or less the same for every open source project, the focus of our contributing document should be on the textual-specific workflows.
For example,

• how does one setup the development environment for textual?
• what's the textual-specific checklist that we need to go through before opening a PR?
• how does one create a snapshot test?

Some of these things have already been answered in #2923, which was linked from the issue #2933 that you are fixing.

So, please take your time to go through #2923 and we're looking forward to your revised PR.
Thanks!

[willmcgugan]

Great work. Thanks!