Added optional dependencies

Author: Christopher Fonnesbeck

README.md

@@ -69,6 +69,8 @@ For students familiar with Git, you may simply clone this repository to obtain a
 * pyzmq
 * tornado
 
+**Optional**: statsmodels, xlrd and openpyxl
+
 For students running the latest version of Mac OS X (10.8), the easiest way to obtain all the packages is to install the [Scipy Superpack](http://bit.ly/scipy_superpack) which works with Python 2.7.2 that ships with OS X.
 
 Otherwise, another easy way to install all the necessary packages is to use Continuum Analytics' [Anaconda](http://docs.continuum.io/anaconda/install.html).