add nbviewer link

Author: TomAugspurger

doc/source/html-styling.ipynb

@@ -8,6 +8,8 @@
     "\n",
     "<p style=\"color: red\">*Provisional: This is a new feature and still under development. We'll be adding features and possibly making breaking changes in future releases. We'd love to hear your [feedback](https://github.com/pydata/pandas/issues).*<p style=\"color: red\">\n",
     "\n",
+    "This document is written as a Jupyter Notebook, and can be viewed or downloaded [here](http://nbviewer.ipython.org/github/pydata/pandas/blob/master/doc/source/html-styling.ipynb).\n",
+    "\n",
     "You can apply **conditional formatting**, the visual styling of a DataFrame\n",
     "depending on the data within, by using the ``DataFrame.style`` property.\n",
     "This is a property that returns a ``pandas.Styler`` object, which has\n",