add details about type hinting to authoring guide (GoogleCloudPlatform#4941)

* add details about type hinting to authoring guide

* Update AUTHORING_GUIDE.md

Good eyes!!

Co-authored-by: Bu Sun Kim <[email protected]>

Co-authored-by: Bu Sun Kim <[email protected]>

Author: leahecole

AUTHORING_GUIDE.md

@@ -218,10 +218,17 @@ Argument types should be documented using Python type annotations as
 introduced in [PEP 484](https://www.python.org/dev/peps/pep-0484/). For example:
 
 ```py
-def hello_world(name: string):
+def hello_world(name: str) -> None:
     print(f"Hello {name}!")
 ```
 
+```py
+def adder(a: int, b: int) -> int:
+    return a+b
+```
+
+Type hinting is enforced using [`flake8-annotations`](https://pypi.org/project/flake8-annotations/), which is enabled by setting the `enforce_type_hints` variable to `True` in the appropriate `noxfile_config.py`. Type hinting is expected in all new samples, and will gradually be added to all compatible existing samples. 
+
 If there is an `Args` section within the function's docstring, consider
 documenting the argument types there as well. For example: