Add the sharedstreets-ref-system repo as a git submodule

[invisiblefunnel]

This unifies the handling of protobufs with the sharedstreets-pbf repo. Updated the readme instructions to reflect the change. This would be a good Makefile target in the future.

Tested this change by deleting sharedstreets/sharedstreets_pb2.py, then regenerating it to ensure an empty diff.

[migurski]

Looks great to me, thank you.