documentation for gomock

[jmhodges]

bazel_gomock has been superseded by the gomock functionality in rules_go itself for a while now.

Since y'all asked to take it over, I've not had a place to send folks to read docs on how to use it. There also have been some changes in functionality over time (the prior version handled import paths differently, iirc). It'd be nice for gomock to get the same docs as the other tools here.

[diedoman]

Not sure if this would help anyone who bounces from jmhodges/bazel_gomock to this repo/issue, but the description of the go_mock function itself can be found here