Add prompb/README (#4222)

Signed-off-by: Henri DF <henridf@gmail.com>

prompb/README.md

@@ -0,0 +1,14 @@
+The compiled protobufs are version controlled and you won't normally need to
+re-compile them when building Prometheus.
+
+If however you have modified the defs and do need to re-compile, run
+`./scripts/genproto.sh` from the parent dir.
+
+In order for the script to run, you'll need `protoc` (version 3.5) in your
+PATH, and the following Go packages installed:
+
+- github.com/gogo/protobuf
+- github.com/gogo/protobuf/protoc-gen-gogofast
+- github.com/grpc-ecosystem/grpc-gateway/protoc-gen-grpc-gateway/
+- github.com/grpc-ecosystem/grpc-gateway/protoc-gen-swagger
+- golang.org/x/tools/cmd/goimports