Updated Linux build info in readme

Remember to init submodules for "qupgrade"

README.md

@@ -65,6 +65,8 @@ To build on Linux:
     sudo apt-get install phonon libqt4-dev libphonon-dev libphonon4 phonon-backend-gstreamer qtcreator libsdl1.2-dev libflite1 flite1-dev build-essential libopenscenegraph-dev
     cd directory
     git clone https://github.com/mavlink/qgroundcontrol.git
+    git submodule init
+    git submodule update
 
 * go to `libs/thirdParty -> libxbee` 
 * Create Library -> Readme file in folder