README.md: mention protopath and protorange packages
README.md: mention protopath and protorange packages

Change-Id: I33045b4356642d433d88112cf0c6e6210dace58e
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/331229
Trust: Joe Tsai <ad9e03fbec35106ba742f446a098eef3d5e31d20@digital-static.net>
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go