Document gRPC roots.pem requirements in top-level README.

[coryan]

The examples/README documents how to setup the roots.pem file for gRPC:

https://github.com/googleapis/google-cloud-cpp/blob/master/google/cloud/bigtable/examples/README.md#configure-grpc-root-certificates

This should be also documented in the bigtable/README.md file, and in the quick start guide, and in the landing page for the reference guide.

[coryan]

Also the current documentation suggests using third_party/grpc which is not really a directory.

[coryan]

Note that this is not needed for Linux, gRPC uses the native trust store in that platform, but it does not on macOS, the PR to fix that was closed without merging:

grpc/grpc#16246

nor does it work on Windows:

grpc/grpc#16571

[coryan]

Actually, this is now done at the README for each quickstart program. Closing.