Add integration readthedocs

[Matthias Wählisch]

Current version cannot be build in Read the Docs:
Extension error: Could not import extension sphinxcontrib.bibtex (exception: No module named 'sphinxcontrib')

Please, send me your RTD account and I will add you to http://rtrlib.readthedocs.io/

[Sebastian Meiling]

fixed, see http://rtrlib.readthedocs.io/en/latest/