Add section in CONTRIBUTING.md on docs changes.

Signed-off-by: Felix Fontein <felix@fontein.de>

CONTRIBUTING.md

@@ -9,6 +9,11 @@ and instructions if you are thinking of helping with the development of SOPS.
   how to install Go [here](https://go.dev/doc/install)
 - Clone the Git repository and switch into SOPS's directory.
 - Run the tests with `make test`. They should all pass.
+- If you modify documentation (RST or MD files), run `make checkdocs` to run
+  [rstcheck](https://pypi.org/project/rstcheck/) and
+  [markdownlint](https://github.com/markdownlint/markdownlint). These should also
+  pass. If you need help in fixing issues, create a pull request (see below) and
+  ask for help.
 - Fork the project on GitHub.
 - Add your fork to Git's remotes:
    - If you use SSH authentication: