cmd/protoc-gen-go: support --help flag
cmd/protoc-gen-go: support --help flag

When someone has no idea what a binary does, it is convention to
pass the "--help" flag. When done, we should point the user
to the devsite documentation on protoc with protoc-gen-go.

Change-Id: I36289a1ae025b9e12521b34362370aba5235a44b
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/302330
Trust: Joe Tsai <ad9e03fbec35106ba742f446a098eef3d5e31d20@digital-static.net>
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go