Add some basic development docs

Author: rfleschenberg

CONTRIBUTING.rst

@@ -0,0 +1,18 @@
+Contributing to pipenv
+======================
+
+To work on pipenv itself, fork the repository and clone your fork to your local
+system.
+
+Now, install the development requirements::
+
+    cd pipenv
+    virtualenv ~/pipenv-venv  # You can use a different path if you like.
+    source ~/pipenv-venv/bin/activate
+    python setup.py develop
+    pipenv install --dev
+
+
+To run the test suite locally::
+
+    pipenv run pytest tests