contributing: Add instructions from PyCon

At PyCon we realised that the virtualenv created in the make file wasn't
used in the subsequent steps in CONTRIBUTING.rst. This change removes the
reference the make and suggesting installing tox directly.
Also changed the explanation that follows to make more sense with this
change

CONTRIBUTING.rst

@@ -158,13 +158,15 @@ but here is a simple overview:
    If you need some help with Git, follow this quick start
    guide: https://git.wiki.kernel.org/index.php/QuickStart
 
-#. Create a development environment
+#. Install tox
+
+   Tox is used to run all the tests and will automatically setup virtualenvs
+   to run the tests in.
    (will implicitly use http://www.virtualenv.org/en/latest/)::
 
-    $ make develop
-    $ source .env/bin/activate
+    $ pip install tox
 
-#. You can now edit your local working copy.
+#. Run all the tests
 
    You need to have Python 2.7 and 3.4 available in your system.  Now
    running tests is as simple as issuing this command::
@@ -177,7 +179,11 @@ but here is a simple overview:
    index where newer (not yet released to pypi) versions of dependencies
    (especially ``py``) might be present.
 
-   To run tests on py27 and pass options (e.g. enter pdb on failure)
+#. You can now edit your local working copy.
+
+   You can now make the changes you want and run the tests again as necessary.
+
+   To run tests on py27 and pass options to pytest (e.g. enter pdb on failure)
    to pytest you can do::
 
     $ python runtox.py -e py27 -- --pdb