P4Runtime Release v1.0.0-rc2

This is the first release we are doing since moving the specification document from gDoc to Github. As a result most of the effort went into translating the spec to a properly-formatted Madoko document.

Changelog:

• specification
  • convert to Madoko (thanks @chrispsommers for the initial conversion work from gDoc!)
  • updated "In Scope" section
  • improved grammar all over document
• protobuf files
  • fix typo in comments
• build
  • added BAZEL rules to build Protobuf / gRPC libraries