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