Adding information about MathJax to notebook install docs.

docs/source/install/install.txt

@@ -348,6 +348,17 b' Tornado'
 The IPython notebook uses the Tornado project [Tornado]_ for its HTTP server and requires
 version 2.0 or greater. Tornado can be installed with ``easy_install tornado``.
 
+MathJax
+-------
+
+The IPython notebook uses the MathJax [MathJax]_ Javascript library for rendering LaTeX
+in web browsers. Because MathJax is large, we don't include it with IPython. We do, however,
+include a utility to aid in downloading MathJax and installing it into the proper location::
+
+    from IPython.external.mathjax import install_mathjax
+    install_mathjax()
+
+
 Browser Compatiblity
 --------------------
 
@@ -372,4 +383,5 b' these features should appear in Internet Explorer 10.'
 .. [pygments] Pygments syntax highlighting. http://pygments.org
 .. [pexpect] Pexpect. http://www.noah.org/wiki/Pexpect
 .. [Tornado] Tornado. http://www.tornadoweb.org/
+.. [MathJax] MathJax. http://www.mathjax.org/