Mention protoc-gen-doc in README.md
Mention protoc-gen-doc in README.md

It's necessary for `go generate` to succeed, because it generates
docs/api.md.

Martin2112/trillian,Martin2112/trillian,Martin2112/trillian,RJPercival/trillian,gdbelvin/trillian,google/trillian,google/trillian,AlCutter/trillian,gdbelvin/trillian,RJPercival/trillian,AlCutter/trillian,gdbelvin/trillian