commit comments: tweak format, add author
Test output here:
https://phabricator.wikimedia.org/T324164#8454933
Adds author for slightly better feature parity with existing gerritbot info. (I'm not sure this matters as much since committer will probably differ from author less often, but I suspect having the name as well as the username is useful signal in a lot of cases.)
Also updates the README.md to link the wiki page.
Bug: T324150