doc: Document how to run external vetters

I like running this tool on Go codebases: https://github.com/dominikh/go-tools/tree/master/cmd/megacheck

I'm not sure the best way to do it. I'm downloading it using go_repository but I'm not sure how to invoke it, whether I have to compile it, or where it installs to.

(I'm new to Bazel but I assume the instructions will be useful to all Go Bazel developers)

Start with the existing go_repository configuration and the linked megacheck tool to determine how Bazel obtains and runs the external vetter. Done means documenting whether compilation is required, how to invoke it, and where the resulting executable is available; no specific file or test is named in the issue.

Written by the indexing model from the issue text.