add comment/explanation for added creds in README

README.md

@@ -80,7 +80,10 @@ The `goSDN` controllers core - also called `nucleus` - is a lightweight library
 that manages principal network domains and provides southbound interface
 operations for managed network elements.
 
-In addition, we provide a simple Northbound-API for the controller [right here](https://code.fbi.h-da.de/danet/gosdn/-/tree/master/controller/api).
+In addition, we provide a simple Northbound-API (gRPC) for the controller [right here](https://code.fbi.h-da.de/danet/gosdn/-/tree/master/controller/api).
+
+The gRPC services can also be reached using HTTP requests via the gRPC-Gateway. The fitting OpenAPI definitions can be found [here](https://code.fbi.h-da.de/danet/gosdn/-/tree/master/api/openapiv2?ref_type=heads). Note, that this is experimental and tested less well. If you want to use the controller in secure mode which implies it's mandatory to login and provide the received token in other requests via the HTTP header with the key-value pair:
+`"authorize: token"`.
 
 ### Principal Networking Domain (PND)