Add TOC to README

Author: ajcr

README.md

@@ -9,11 +9,25 @@ Since pandas is a large library with many different specialist features and func
 
 The exercises are loosely divided in sections. Each section has a difficulty rating; these ratings are subjective, of course, but should be a seen as a rough guide as to how elaborate a solution is required.
 
-The official documentation for pandas is very useful and very extensive. Good places get a broader overview of pandas are
+| Section Name  | Description |  Difficulty |
+| ------------- | ------------- | ------------- |
+| Importing pandas  | Getting started and checking your pandas setup  | Easy |
+| DataFrame basics  | A few of the fundamental routines for selecting, sorting, adding and aggregating data in DataFrames  | Easy  |
+| DataFrames: beyond the basics  | Slightly trickier: you may need to combine two or more methods to get the right answer  | Medium |
+| DataFrames: harder problems  | These might require a bit of thinking outside the box...  | Hard |
+| Series and DatetimeIndex  | Exercises for creating and manipulating Series with datetime data  | Easy/Medium |
+| Cleaning Data  | Making a DataFrame easier to work with  | Easy/Medium |
+| Using MultiIndexes  | Go beyond flat DataFrames with additional index levels  | Medium |
+| Minesweeper | Generate the numbers for safe squares in a Minesweeper grid | Hard |
+
+
+The official documentation for pandas is very useful and very extensive. Good places get a broader overview of pandas are:
 
 - [10 minutes to pandas](http://pandas.pydata.org/pandas-docs/version/0.17.0/10min.html)
 - [pandas basics](http://pandas.pydata.org/pandas-docs/version/0.17.0/basics.html)
 - [tutorials](http://pandas.pydata.org/pandas-docs/stable/tutorials.html)
 - [cookbook and idioms](http://pandas.pydata.org/pandas-docs/version/0.17.0/cookbook.html#cookbook)
 
-\* the list of exercises is not complete! Pull requests or suggestions for additional exercises, corrections and improvements are welcomed.
+Good luck solving the puzzles!
+
+\* the list of puzzles is not complete! Pull requests or suggestions for additional exercises, corrections and improvements are welcomed.