Update core contributors list.

I've sorted our usernames alphabetically.

Author: drvinceknight

README.rst

@@ -131,8 +131,10 @@ contributors <https://github.com/Axelrod-Python/Axelrod/graphs/contributors>`_.
 The Core developers of the project are:
 
 - `drvinceknight <https://github.com/drvinceknight>`_
+- `gaffney2010 <https://github.com/gaffney2010>`_
 - `marcharper <https://github.com/marcharper>`_
 - `meatballs <https://github.com/meatballs>`_
+- `nikoleta-v3 <https://github.com/Nikoleta-v3>`_
 
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