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

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_.