Add notes on initialising Pyrr

Author: adamlwgriffiths

README.md

@@ -44,12 +44,20 @@ PyGLy requires the following software:
    * NumPy
    * Pyrr (https://github.com/adamlwgriffiths/Pyrr)
 
+
+Install Pyrr
+```
+git submodule init
+git submodule update
+```
+
 Install PyGLy depedencies:
 ```
 pip install -r requirements.txt
 ```
 
 Windows users can stop there.
+
 Mac OS-X users need to continue to install more dependencies:
 ```
 pip install -r requirements-osx.txt