Add setup instructions to CONTRIBUTING.md (#93)

CONTRIBUTING.md

@@ -2,6 +2,20 @@
 
 Contributions are always welcome, both reporting issues and submitting pull requests!
 
+### Setup
+
+Follow these steps to run the shopify_python tests:
+
+```bash
+pyenv local 3.6.3  # this is the version running on the 3.6 CI nodes
+python -m venv ~/.pyenv/virtualenvs/shopify_python # or wherever you want the env to be
+source ~/.pyenv/virtualenvs/shopify_python/bin/activate
+make install
+make run_tests
+```
+
+If all went well, you should see a number of successful tests.
+
 ### Reporting issues
 
 Please make sure to include any potentially useful information in the issue, so we can pinpoint the issue faster without going back and forth.