Update README with chapters 5 and 6

Author: jvns

README.md

@@ -21,6 +21,8 @@ Table of Contents
 * [Chapter 2: Selecting data & finding the most common complaint type](http://nbviewer.ipython.org/github/jvns/pandas-cookbook/blob/master/cookbook/Chapter%202%20-%20Selecting%20data%20&%20finding%20the%20most%20common%20complaint%20type.ipynb)
 * [Chapter 3: Which borough has the most noise complaints? (or, more selecting data)](http://nbviewer.ipython.org/github/jvns/pandas-cookbook/blob/master/cookbook/Chapter%203%20-%20Which%20borough%20has%20the%20most%20noise%20complaints%3F%20%28or%2C%20more%20selecting%20data%29.ipynb)
 * [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)