clarify how to add a proto file without including its autogen'd stubs

[vchudnov-g]

I was trying to include a new schema/ proto file in #1573 (and everything worked locally) but the GitHub presubmits kept failing unless the corresponding generated pb.go file was included in the PR.

I believe the intent is for the pb.go files to not need to be included in PRs, so we should make it clear how to do that while getting presubmits to pass. If we do need to include the generated files in PRs, we should clarify that.

[vchudnov-g]

As @noahdietz points out, the issue might lie with this sequence of steps. In the PR, I only tried swapping two of those, but maybe we need to reorder that whole sequence.

The error happens within the "Regenerate Sources" step.