Describe GitHub workflow: create a patch from pull request to r-devel/r-svn

[krlmlr]

I'd argue that with https://github.com/r-devel/r-svn/ we no longer need to understand SVN to submit a patch to the Bugzilla system. It also brings the huge advantage that the patch is tested by GitHub Actions on all/most/many operating systems. Would you support splitting the patch submission chapter into two parts -- the SVN way and the GitHub way?

This is what the GitHub way roughly looks like:

• fork and clone repository
• create a branch, implement your changes
• push
• create a pull request
• wait for CI builds
• if CI builds fail, push more commits
• rinse and repeat until builds succeed
• send link to PR and link to patch to Bugzilla, the patch can be created directly from GitHub
• R core can review the patch on GitHub or in its text form
• If changes are needed, you can push again on GitHub, links will update automatically

CC @jeroen @lionel-.