Add Contributors documentation to projects, and consider switching to GitHub reviews from Gerrit

[whesse]

To encourage contributions from other authors, either instructions for how to create Gerrit CLs for changes should be added,
or the repository could be switched to use GitHub reviews.

As @devoncarew suggested when suggesting changes with a PR:
"If PR won't work here, I can recreate this work as a gerrit CL. However, it would be nice to either take PRs, or, add some brief contributing instructions to the repo to describe how to author changes.

You might consider flipping the system of record for the codebase, as that would allow us to do things like use github actions to validate the code."

[devoncarew]

consider switching to GitHub reviews from Gerrit

One things that would be possible - if this repo moves closer to using github as the system of record - is to auto-deploy the app to firebase when changes land on master. We've set that up on one other Flutter web + firebase app in the dart-lang/ org; I can provide pointers if that's interesting.