Add C++ gRPC. (README.google only in this diff.)
Add C++ gRPC. (README.google only in this diff.)

Summary:
This is the README.google for C++ gRPC.
Adding the code is too much for Phabricator to bear
so I'll do it by pushing directly to github should
this diff be approved.

Reviewers: schroederc

Reviewed By: schroederc

Maniphest Tasks: T30

Differential Revision: https://phabricator-dot-kythe-repo.appspot.com/D157

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