Add count of strategies as a doctest.

Author: drvinceknight

docs/index.rst

@@ -49,6 +49,12 @@ validity and reproducibility of results!
 Quick start
 -----------
 
+Count the number of available players::
+
+    >>> import axelrod as axl
+    >>> len(axl.strategies)
+    236
+
 Create matches between two players::
 
     >>> import axelrod as axl

docs/tutorials/contributing/strategy/adding_the_new_strategy.rst

@@ -24,6 +24,8 @@ above with your new strategy.
 
 Once you have done that, you need to add the class itself to the
 :code:`all_strategies` list (in :code:`axelrod/strategies/_strategies.py`).
+You will also need to increment the doctest in
+:code:`axelrod/docs/index.rst`.
 
 Finally, if you have created a new module (a new :code:`<strategy.py>` file)
 please add it to the :code:`docs/references/all_strategies.rst` file so that it