Move PGV documentation back into the root README (#1012)

In #938 we moved
information about PGV out of the main readme and into a separate file.
At the same time, we inlined more information about Protovalidate into
the main PGV readme, including directly linking to per-language
Protovalidate repositories.

Since then, we have had many instances of confusion on the Protovalidate
issue tracker with users asking question about PGV (in part because I
think people didn't realize this was a separate project).

To alleviate this confusion, this change moves PGV documentation back to the
main readme while maintaining a clear note at the top about the status
of this project as well as the existence of Protovalidate.