Add note about autogeneration of plugin docs (#395)

# References and relevant issues
Closes #391 

# Description
Adds a note about how the plugin docs are generated from npe2 templates
through the prep_docs.py script.

docs/developers/contributing/documentation/index.md

@@ -115,6 +115,20 @@ want to contribute. The paths are listed in parentheses below.
 - [**Getting started**](getting-started) (in [`napari/docs/tutorials/fundamentals`](https://github.com/napari/docs/tree/main/docs/tutorials/fundamentals)):
   these documents are a mix of tutorials and how-tos covering the fundamentals of installing and working with napari for beginners
 
+```{admonition} Plugin documentation
+:class: tip
+
+Some of the source files for the [Plugin documentation](plugins-index) are
+autogenerated from sources in the [napari/npe2](https://github.com/napari/npe2)
+repository. Any edits should be made in the `napari/npe2` repo, and not to the
+copy you may have in the `napari/docs' folder. These files' names start with the
+`_npe2_` prefix and are located under the `docs/plugins` folder. They will be
+generated when the documentation is built and the
+[`prep_docs.py`](https://github.com/napari/docs/blob/main/docs/_scripts/prep_docs.py)
+script is run. See our
+[Makefile](https://github.com/napari/docs/blob/main/Makefile) for more details.
+```
+
 The [**Examples gallery**](gallery) sources are in the [main `napari/napari` repository](https://github.com/napari/napari/tree/main/examples)
 and show code examples of how to use napari.