Update README.rst

Add `Install pytest-django` section

Author: dduong42

README.rst

@@ -26,6 +26,13 @@ pytest-django allows you to test your Django project/applications with the
 * `Issue tracker <http://github.com/pytest-dev/pytest-django/issues>`_
 * `Python Package Index (PyPI) <https://pypi.python.org/pypi/pytest-django/>`_
 
+Install pytest-django
+---------------------
+
+::
+
+    pip install pytest-django
+
 Why would I use this instead of Django's `manage.py test` command?
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