Makefile: Document and organize into sections
Makefile: Document and organize into sections

Add documentation and move protobuf target into Build section

Signed-off-by: Euan Harris <c3b6e83069c8e9e3af49a38fec6026be89559638@docker.com>

docker/libnetwork,mahak/libnetwork,medallia/libnetwork,docker/libnetwork,docker/libnetwork,mahak/libnetwork,medallia/libnetwork,mahak/libnetwork,medallia/libnetwork