Create CONTRIBUTING.md guidelines

[alice-i-cecile]

What problem does this solve or what need does it fill?

We should have one central place to point new (and old) contributors when they have questions about the contribution process, especially when it come to PRs.

I think this is particularly valuable for contributors who are new to Rust or open source development. This is more likely to be true with Bevy than other projects, due to its accessibility and broad interest.

What solution would you like?

Create a CONTRIBUTING.md file.

Current steps for making a PR:

1. Fork the repo.
2. Make changes locally.
3. Run cargo fmt. Run cargo +nightly fmt.
4. Run cargo clippy. Take the clippy command from the workflows folder & run the same command.
5. Submit a pull request with a descriptive title and any important context about the changes.

What alternative(s) have you considered?

Continue to pass this information on one contributor at a time, or trust that they're experienced to know how to do so.

Additional context

Prompt for this issue.

[rparrett]

Should that be cargo +nightly fmt?

[CleanCut]

Like this? #1309

[CleanCut]

Actually resolved by bevyengine/bevy-website#85 -- the place to point to is https://bevyengine.org/learn/book/contributing/code/

#1309 continues down a different avenue...