Added readme file

hivemind/tavern/README.md

+# Usage
+
+To run tavern test:
+1. Enter `tavern` directory
+2. Edit `common.yaml` and set variables to desired values
+3. Install tavern/pytest `pip install tavern[pytest]`
+4. Run tests: `PYTHONPATH=$PYTHONPATH:$(pwd) py.test -v`