Add comment for proto in Makefile

Makefile

@@ -21,6 +21,7 @@ proto:
 		exit 1; \
 	fi
 	go get -u -v github.com/golang/protobuf/protoc-gen-go
+	# use $$dir as the root for all proto files in the same directory
 	for dir in $$(git ls-files '*.proto' | xargs -n1 dirname | uniq); do \
 		protoc -I $$dir --go_out=plugins=grpc:$$dir $$dir/*.proto; \
 	done