Add CONTRIBUTING

Author: ForeverWintr

CONTRIBUTING.md

@@ -0,0 +1,10 @@
+To develop metafunctions, first create a virtual environment, then install the project in development mode:
+
+    $ python3 -m venv venv
+    $ . venv/bin/activate
+    $ pip install -e .
+
+The project uses Tox to run tests against multiple python versions. To run the tests:
+
+    $ pip install tox
+    $ tox