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 #379 (comment)

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.