Move the online documentation to Read the Docs

[RKrahl]

The online documentation is currently hosted at http://pythonhosted.org/pytest-dependency/. But pythonhosted.org has essentially been closed down without a warning, without any option to update the documentation. We urgently need to move to a new host. https://readthedocs.org/ seem to be a valid option.

[RKrahl]

I successfully moved the documentation to https://pytest-dependency.readthedocs.io/ and updated the link in the README. As far as pytest-dependency is concerned, this is done.

However, there seem to be now way to update the Package Documentation link in the PyPI page or to install a redirection from the outdated documentation on pythonhosted.org, see pypi/warehouse#582. Leave this issue open as a reminder until this is fixed in PyPI.

[RKrahl]

Now I deleted the docs at pythonhosted.org and added project_urls to setup.py. I guess that is all what can be done.