Update contribution guidelines around type annotations (#1063)

Since #1061 has merged rustworkx is now fully typed with mypy. This
means that adding type annotations is no longer optional and is now
required for all new function. This commit updates the contribution
guidelines to indicate type annotations are required now. The details
are also updated a bit to provide a bit more detail on what's required
when adding type annotations to rustworkx.

Co-authored-by: Ivan Carvalho <[email protected]>

CONTRIBUTING.md

@@ -274,19 +274,41 @@ cargo doc --open
 
 ### Type Annotations
 
-If you have added new methods or changed method signatures, we encourage you to add annotations for 
-those methods in stub files. The stub files are in the `rustworkx` directory and have a `.pyi` file extension.
-They contain annotated signatures for Python functions, stripped of their implementation.
-
-While this step is optional, it is very helpful for end-users. Adding annotations lets users type check
-their code with [mypy](http://mypy-lang.org/), which can be helpful for finding bugs.
+If you have added new methods, functions, or classes, and/or changed any
+signatures, type anotations for Python are required to be included in a pull
+request. Type annotations are added using type
+[stub files](https://typing.readthedocs.io/en/latest/source/stubs.html) which
+provide type annotations to python tooling which use type annotations. The stub
+files are in the `rustworkx/` directory and have a `.pyi` file extension. They
+contain annotated signatures for Python functions, stripped of their
+implementation. You can find more details on typing in Python at:
+
+ * https://mypy.readthedocs.io/en/stable/
+ * https://typing.readthedocs.io/en/latest/
+ * https://docs.python.org/3/library/typing.html
+
+Having type annotations is very helpful for Python end-users. Adding
+annotations lets users type check their code with [mypy](http://mypy-lang.org/),
+which can be helpful for finding bugs when using rustworkx.
 
 Just like with tests for the code, annotations are also tested via tox.
 
 ```
 tox -estubs
 ```
 
+One important thing to note is that if you're adding a new function to the Rust
+module you will need to ensure that the signature with annotations is added to
+`rustworkx/rustworkx.pyi`. Then it is also necessary to re-export the annotation
+by adding an import line to `rustworkx/__init__.pyi` in the form:
+
+```python
+from .rustworkx import foo as foo
+```
+
+which ensures that mypy is able to find the type annotations when users import
+from the root `rustworkx` package (which is the most common access pattern).
+
 ### Release Notes
 
 It is important to document any end user facing changes when we release a new