docs: Add documentation for hacking on Pillow

It's not clear from the existing documentation how to set up a local
environment for hacking on pillow.  Add some docs for this purpose so
that new contributors can hit the ground running more quickly.

docs/hacking.rst

@@ -0,0 +1,31 @@
+===================
+ Hacking on Pillow
+===================
+
+Found a bug?  Want to submit a feature?  This page will help you get
+started with doing development on Pillow.
+
+First, get the source::
+
+    $ git clone https://github.com/python-pillow/Pillow.git
+    $ cd Pillow
+
+Install pre-requisits::
+
+    $ virtualenv env
+    $ . env/bin/activate
+    $ pip install nose
+
+Build the ``develop`` target::
+
+    $ python setup.py develop
+
+Check that all the tests pass in your environment::
+
+    $ nosetests -vx Tests/test_*.py
+
+Assuming all the tests pass successfully you're all set!  Go ahead and hack
+away.  When you're ready to submit your work back to the Pillow project,
+just `fork and submit a pull request
+<https://help.github.com/articles/using-pull-requests/>`_ as usual.  You
+will probably be asked to submit a new test along with your pull request.

docs/index.rst

@@ -45,6 +45,7 @@ Pillow is the friendly PIL fork by `Alex Clark and Contributors <https://github.
    handbook/index.rst
    reference/index.rst
    porting.rst
+   hacking.rst
    about.rst
    releasenotes/index.rst