User's Guide: Add "Contributing to documentation" documentation

[samsrabin]

What sort(s) of documentation issue is this?

• Something is missing.
• Something is (or might be) incorrect or outdated.
• Something is confusing.
• Something is broken.

What part(s) of the documentation does this concern?

• Technical Note (science and design of the model)
• User's Guide (using the model and related tools)
• Somewhere else (e.g., README file, tool help text, or code comment): Please specify
• I don't know

Describe the issue

With many new people contributing to the docs over the coming months, it will be useful to document best practices for this work.

[wwieder]

This would be very helpful to have

[samsrabin]

@wwieder Could you clarify exactly what we're looking for here? There is already some material about opening a PR. Are you saying it'd be good to have a subsection specifically called something like "Contributing your changes"?

Note that I'm planning (#3151) to add a style guide, which will also help.

[wwieder]

I might put this question back on you, @samsrabin. You've been frustrated with external documentation PRs that were coming in. What would help alleviate this frustration? Specifically it could be related to how contributors commit your changes and submit a pull request? Would a quick description of how to do this be helpful in the docs to help train new contributors how we'd like PRs to come it?

[samsrabin]

Thanks, that clarifies things! I think I'll also add a comment in our PR template saying, "If this is a documentation change, please review the guidelines at [link]."