update docstring to include binderhub
MODIFY
repo2docker/app.py
repo2docker/app.py
"""repo2docker: convert git repositories into jupyter-suitable docker images
-Images produced by repo2docker can be used with Jupyter notebooks standalone or via JupyterHub.
+Images produced by repo2docker can be used with Jupyter notebooks standalone,
+with BinderHub, or with JupyterHub.
Usage:

update docstring to include binderhub