Point to jupyter.org's notebook viewer instead of github.com's becaus…

…e github's does not display math formulas correctly

README.md

@@ -10,8 +10,9 @@ Simply open the [Jupyter](http://jupyter.org/) notebooks you are interested in:
 
 * using [Binder](http://mybinder.org/): [![Binder](http://mybinder.org/badge.svg)](http://mybinder.org/repo/ageron/ml-notebooks)
     * this let's you experiment with the code examples
-* or directly within github (start at [index.ipynb](https://github.com/ageron/ml-notebooks/blob/master/index.ipynb))
-* or by cloning this repository and running Jupyter locally.
+* or using [jupyter.org's notebook viewer](http://nbviewer.jupyter.org/github/ageron/ml-notebooks/blob/master/index.ipynb)
+    * note: [github.com's notebook viewer](https://github.com/ageron/ml-notebooks/blob/master/index.ipynb)) also works but it is slower and the math formulas are not displayed correctly
+* or by cloning this repository and running Jupyter locally
     * if you prefer this option, follow the installation instructions below.
 
 # Installation