update prerequisites to include ipywidgets dev master

README.md

@@ -56,6 +56,7 @@ the browser.
 
 - Jupyter notebook version 4.2 or later
 - NodeJS (preferably version 5 or later) and npm
+- Optionally (if you plan to use ipython widgets in the notebook): ipywidgets dev master installed from https://github.com/ipython/ipywidgets
 
 #### Installation