Update committer guide around merging from GitHub

The committer guide in https://hbase.apache.org/book.html#_guide_for_hbase_committers seems to focus mostly on committing patch files that were uploaded to jira. There are some brief mentions of GitHub, but I think these days GitHub is the primary mode of contributing. Correct me if I'm wrong.

If so, I think we should update the docs with instructions from a GitHub-centric workflow. 

I'd be happy to update our guide, but I will need help on collecting the basic steps since I don't know how to do it myself yet.