Suggestion: Add a pull request template (to discourage PRs)

[roybaer]

Since GitHub apparently does not want to give people the ability to disable the pull request tab altogether, the next best option might be to add a pull request template that pre-fills the PR description with a friendly note that redirects people to the mailing list.

This would avoid the current scenario where some people spend time preparing a pull request just to have it closed by a bot, because they would be greeted with the content of that bot message as soon as they press the "New pull request" button, i.e. before they create it.

See https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/creating-a-pull-request-template-for-your-repository