message: update contributing.md
  Uses pip instead to setup superset.
diff: @@ -173,7 +173,7 @@ Check the [OS dependencies](https://superset.incubator.apache.org/installation.h
  source env/bin/activate
  # install for development
  - python setup.py develop
  + pip install -e .
  # Create an admin user
  fabmanager create-admin --app superset
  