Document GitHub API rate limit considerations

We're using unauthenticated API requests to GitHub which is subject to a tighter rate limit than an authenticated rate limit. In normal usage, this isn't a problem, but it's possible to spam update requests and get rate limited. We should make sure to document this.

The issue names no documentation file, test, or entry point. Locate the project's existing documentation and GitHub API guidance first; done means documenting the tighter unauthenticated rate limit and the risk of repeated update requests causing rate limiting.

Written by the indexing model from the issue text.