Added Jeff Coffler to contributors.yaml.
Added Jeff Coffler to contributors.yaml.

Review: https://reviews.apache.org/r/58015/

andschwa/mesos,reneploetz/mesos,jpeach/mesos,verizonlabs/mesos,zmalik/mesos,gsantovena/mesos,andschwa/mesos,asamerh4/mesos,Gilbert88/mesos,chhsia0/mesos,kaysoky/mesos,reneploetz/mesos,abudnik/mesos,gsantovena/mesos,kaysoky/mesos,abudnik/mesos,reneploetz/mesos,dforsyth/mesos,verizonlabs/mesos,neilconway/mesos,asamerh4/mesos,kaysoky/mesos,verizonlabs/mesos,gsantovena/mesos,dforsyth/mesos,andschwa/mesos,chhsia0/mesos,reneploetz/mesos,Gilbert88/mesos,kaysoky/mesos,dforsyth/mesos,zmalik/mesos,craimbert/mesos,jpeach/mesos,dforsyth/mesos,chhsia0/mesos,zmalik/mesos,gsantovena/mesos,craimbert/mesos,asamerh4/mesos,jpeach/mesos,zmalik/mesos,neilconway/mesos,verizonlabs/mesos,abudnik/mesos,dforsyth/mesos,verizonlabs/mesos,Gilbert88/mesos,zmalik/mesos,Gilbert88/mesos,neilconway/mesos,reneploetz/mesos,dforsyth/mesos,gsantovena/mesos,dforsyth/mesos,neilconway/mesos,chhsia0/mesos,gsantovena/mesos,gsantovena/mesos,andschwa/mesos,chhsia0/mesos,asamerh4/mesos,verizonlabs/mesos,Gilbert88/mesos,asamerh4/mesos,jpeach/mesos,Gilbert88/mesos,craimbert/mesos,kaysoky/mesos,jpeach/mesos,abudnik/mesos,abudnik/mesos,verizonlabs/mesos,andschwa/mesos,kaysoky/mesos,zmalik/mesos,Gilbert88/mesos,craimbert/mesos,andschwa/mesos,chhsia0/mesos,abudnik/mesos,reneploetz/mesos,chhsia0/mesos,andschwa/mesos,craimbert/mesos,neilconway/mesos,asamerh4/mesos,zmalik/mesos,neilconway/mesos,reneploetz/mesos,abudnik/mesos,jpeach/mesos,neilconway/mesos,kaysoky/mesos,jpeach/mesos,craimbert/mesos,craimbert/mesos,asamerh4/mesos