chore: Update docs for 2.0

CONTRIBUTING.md

@@ -35,8 +35,11 @@ You want to make sure where in the file you add your exercise. If you're not sur
 ```diff
   ...
 + [[exercises]]
++ name = "yourTopicN"
 + path = "exercises/yourTopic/yourTopicN.rs"
 + mode = "compile"
++ hint = """
++ Some kind of useful hint for your exercise."""
   ...
 ```
 
@@ -125,4 +128,4 @@ BREAKING CHANGE: This has to be done because lorem ipsum dolor
 Once you open a Pull Request, it may be reviewed or labeled (or both) until
 the maintainers accept your change. Then, [bors](https://github.com/bors) will
 run the test suite with your changes and if it's successful, automatically
-merge it in!
+merge it in!