MobileFrontend should include a CONTRIBUTORS.MD file which potential volunteer contributors can read to understand where and how they can contribute.

This document should align with the current support level for the project overall ("Active Maintenance" overall), the feature-level ownership described in OWNERS.md, and with the ultimate goal of "strategic sunsetting" (no new features here, gradually reduce the scope of what this extension is responsible for, etc).

Here is what a good contributor doc looks like for an active project: https://gerrit.wikimedia.org/r/c/mediawiki/extensions/ReaderExperiments/+/1204722. We'll want to tailor this to the considerations above.