6611 update docstring format guide (#6780)

part of #6611



### Types of changes
<!--- Put an `x` in all the boxes that apply, and remove the not
applicable items -->
- [x] Non-breaking change (fix or new feature that would not break
existing functionality).
- [ ] Breaking change (fix or new feature that would cause existing
functionality to change).
- [ ] New tests added to cover the changes.
- [ ] Integration tests passed locally by running `./runtests.sh -f -u
--net --coverage`.
- [ ] Quick tests passed locally by running `./runtests.sh --quick
--unittests --disttests`.
- [ ] In-line docstrings updated.
- [ ] Documentation updated, tested `make html` command in the `docs/`
folder.

Signed-off-by: Wenqi Li <wenqil@nvidia.com>

CONTRIBUTING.md

@@ -151,6 +151,10 @@ make html
 ```
 The above commands build html documentation, they are used to automatically generate [https://docs.monai.io](https://docs.monai.io).
 
+The Python code docstring are written in
+[reStructuredText](https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html) and
+the documentation pages can be in either [reStructuredText](https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html) or [Markdown](https://en.wikipedia.org/wiki/Markdown).  In general the Python docstrings follow the [Google style](https://google.github.io/styleguide/pyguide.html#38-comments-and-docstrings).
+
 Before submitting a pull request, it is recommended to:
 - edit the relevant `.rst` files in [`docs/source`](./docs/source) accordingly.
 - build html documentation locally