Update README.md

Author: jvns

README.md

@@ -23,6 +23,7 @@ Table of Contents
 * [Chapter 4: Find out on which weekday people bike the most with groupby and aggregate](http://nbviewer.ipython.org/github/jvns/pandas-cookbook/blob/master/cookbook/Chapter%204%20-%20Find%20out%20on%20which%20weekday%20people%20bike%20the%20most%20with%20groupby%20and%20aggregate.ipynb)
 * [Chapter 5: Combining dataframes and scraping Canadian weather data](http://nbviewer.ipython.org/github/jvns/pandas-cookbook/blob/master/cookbook/Chapter%205%20-%20Combining%20dataframes%20and%20scraping%20Canadian%20weather%20data.ipynb)
 * [Chapter 6: String operations! Which month was the snowiest?](http://nbviewer.ipython.org/github/jvns/pandas-cookbook/blob/master/cookbook/Chapter%206%20-%20String%20operations%21%20Which%20month%20was%20the%20snowiest%3F.ipynb)
+* [Chapter 7: Cleaning up messy data](http://nbviewer.ipython.org/github/jvns/pandas-cookbook/blob/master/cookbook/Chapter%207%20-%20Cleaning%20up%20messy%20data.ipynb)