notes_for_developers.rst: Mention protoc limitation with relative import paths and workaround

Before raising this MR, consider whether the following are required, and complete if so:

• Unit tests
• Metrics
• Documentation update(s)

If not required, please explain in brief why not.

Description

This adds a mention of the need to manually adjust import paths when generating protobufs to be used by BuildGrid.

Changes proposed in this merge request:

• Add a note in docs/source/developer/notes_for_developers.rst about having to adjust the modules generated by protobuf in order to make them relative to buildgrid._protos