Closes#3497.
I added the rust API guidelines (https://rust-lang.github.io/api-guidelines/about.html) to the `CONTRIBUTING.md` file.
## Note
As noted in #3497 we should note any areas where we deliberately disagree as they arise. If we start adding these areas it might be a good idea to remove the mention of the `API guidelines` in the `CONTRIBUTING.md` file and move it to the `engine_style_guide.md`. That way we still have the connection between the `CONTRIBUTING.md` and the `API guidelines`, but we have more "space" to work with and can go into more detail about what we agree and disagree on.
For now I would leave it this way, because it's one less click to get to the guidelines.
Co-authored-by: KDecay <KDecayMusic@protonmail.com>
[**RENDERED**](https://github.com/alice-i-cecile/bevy/blob/better-contributing/CONTRIBUTING.md)
Improves #910. As discussed in #1309, we'll need to synchronize content between this and the Bevy website in some way (and clean up the .github file perhaps?).
I think doing it as a root-directory file is nicer for discovery, but that's a conversation I'm interested in having.
This document is intended to be helpful to beginners to open source and Bevy, and captures what I've learned about our informal practices and values.
Reviewers: I'm particularly interested in:
- opinions on the items **What we're trying to build**, where I discuss some of the project's high-level values and goals
- more relevant details on the `bevy` subcrates for **Getting oriented**
- useful tricks and best practices that I missed
- better guidance on how to contribute to the Bevy book from @cart <3
