Consider automating additions to CONTRIBUTORs

[bpkroth]

          Later we should consider to automate this and/or add additional authors from the history.

Originally posted by @bpkroth in https://github.com/cmu-db/benchbase/pull/379#discussion_r1392927712

At the very least a small check that looks to see if a PR includes the current author in the CONTRIBUTORs file already and suggest that they add it manually if not would be a decent start I think.