Added API guidelines to CONTRIBUTING.md (bevyengine#3646)

Closes bevyengine#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 bevyengine#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 <[email protected]>

.github/contributing/engine_style_guide.md

@@ -1,6 +1,10 @@
 # Style guide: Engine
 
-For more advice on contributing to the engine, see the [relevant section](../../CONTRIBUTING.md#Contributing-your-own-ideas) of CONTRIBUTING.md.
+## Contributing
+
+For more advice on contributing to the engine, see the [relevant section](../../CONTRIBUTING.md#Contributing-your-own-ideas) of `CONTRIBUTING.md`.
+
+## General guidelines
 
 1. Prefer granular imports over glob imports of `bevy::prelude::*` and `bevy::sub_crate::*`.
 2. Use a consistent comment style:
@@ -10,3 +14,25 @@ For more advice on contributing to the engine, see the [relevant section](../../
    4. Use \`variable_name\` code blocks in comments to signify that you're referring to specific types and variables.
    5. Start comments with capital letters. End them with a period if they are sentence-like.
 3. Use comments to organize long and complex stretches of code that can't sensibly be refactored into separate functions.
+
+## Rust API guidelines
+
+As a reference for our API development we are using the [Rust API guidelines][Rust API guidelines]. Generally, these should be followed, except for the following areas of disagreement:
+
+### Areas of disagreements
+
+Some areas mentioned in the [Rust API guidelines][Rust API guidelines] we do not agree with. These areas will be expanded whenever we find something else we do not agree with, so be sure to check these from time to time.
+
+> All items have a rustdoc example
+
+- This guideline is too strong and not applicable for everything inside of the Bevy game engine. For functionality that requires more context or needs a more interactive demonstration (such as rendering or input features), make use of the `examples` folder instead.
+
+> Examples use ?, not try!, not unwrap
+
+- This guideline is usually reasonable, but not always required.
+
+> Only smart pointers implement Deref and DerefMut
+
+- Generally a good rule of thumb, but we're probably going to deliberately violate this for single-element wrapper types like `Life(u32)`. The behavior is still predictable and it significantly improves ergonomics / new user comprehension.
+
+[Rust API guidelines]: https://rust-lang.github.io/api-guidelines/about.html