Add instruction for submodules

python-intro/README

@@ -61,6 +61,12 @@ This will create a set of slides to be viewed in a browser:
 
     ipython nbconvert reveal python-intro.ipynb
 
+Make sure you have the reveal.js submodule repository initialized, otherwise
+you won't have any styling and navigation in your slides:
+
+    git submodule init
+    git submodule update
+
 As a reference for students, you could also create a full HTML dump:
 
     ipython nbconvert full_html python-intro.ipynb