Document / automate(?) building generated files from protobuf specs

[tseaver]

See discussion in #1476. It would be good to have a clean way to refresh protobufs from their upstream sources and rebuild generated files, without relying on lots of hand-rolled environment setup.

[dhermes]

@nathanielmanistaatgoogle What is the easiest way to get grpc_python_plugin installed?

[nathanielmanistaatgoogle]

@dhermes at the moment it's still to build and install all of gRPC from source but @soltanmm is soon going to be working on bundling it as its own Python package (or bundling it into the one Python package we already have).

[dhermes]

👍 We wouldn't really care if #1384 became a thing.