Explain "Allow edits from maintainers" within the contributing guide

[dbeatty10]

Describe the documentation update

It is easier to collaborate on Pull Requests on user-owned forks when "Allow edits from maintainers" is selected by the contributor.

So we should add a note to our contributing guide that says something to that effect.

Additional context

We've had cases where this is not enabled. This means that a maintainer can't push changes and all changes need to be initiated by the contributor. To make changes and merge a PR expediently, a new PR may need to be opened by the maintainer (preserving commits if possible, of course). In that scenario, the main downside is the PR that is merged doesn't have the handle of the contributor, so may not be fully credited in the changelog. The original PR would need to be closed as well.

Who will this benefit?

This makes collaboration easier.