Document parameters in README
Document parameters in README

Adds a section to the README explaining the parameters accepted by the
plugin.

Signed-off-by: David Symonds <7f6bb564ed9e580d4f0aa90d53b34578e5d24cb9@golang.org>

adnaan/protobuf,skyportsystems/go-protobuf,tamird/protobuf-2,fuchsia-mirror/third_party-github.com-golang-protobuf,trdata/protobuf,golang/protobuf,golang/protobuf,trdata/protobuf,tamird/protobuf-2,skyportsystems/go-protobuf,fuchsia-mirror/third_party-github.com-golang-protobuf