Update readme with information about submodules

See merge request !311

README.md

@@ -2,6 +2,11 @@
 
 `goSDN` is a prototypical approach to build a model driven multi-vendor SDN controller.
 
+This repository contains submodules, therefore it must be cloned with
+`git clone --recurse-submodules git@code.fbi.h-da.de:danet/gosdn.git`.
+If you have cloned the repository without `--recurse-submodules` simply run `git submodule update --init --recursive`
+to get all submodules.
+
 # Table of Contents
 
 - [Overview](#overview)