Add a requirements.txt or environment.yml for repo2docker support

[westurner]

Add a requirements.txt or environment.yml for repo2docker and binderhub (mybinder.org,) support

• repo2docker

  repo2docker is the tool used by BinderHub to build images on demand.

  • docs: https://repo2docker.readthedocs.io/en/latest/
• add a /requirements.txt or an /environment.yml (or .binder/[...])
  • REES https://repo2docker.readthedocs.io/en/latest/specification.html
• add a binderhub README badge: https://mybinder.org/