Add mybinder.org link to README

[betatim]

mybinder.org is a free infrastructure operated by Project Binder. The software used to operate it is open-source, based on JupyterHub and even the deployment/operations are performed in the open by a team of volunteers.

Since #369 the stability of mybinder.org has improved a lot (but then I'd say that because I help run it ...) so I thought we could rediscuss this. What do you think?