This is an initial partial description of the plugin protocol focused
mainly on explaining the purpose of the .proto files. In subsequent
updates we will also document the negotiation protocol, etc.
For this first pass the goal was just to publish some information that was
already available in an internal design document so that it's visible to
SDK implementers. It is focused on the .proto versioning strategy because
that was the main topic of the internal design documentation this was
based on.
