added the necessary requirements to build on RTD
added the necessary requirements to build on RTD

 * docs/rtd-pip-requirements: added the necessary requirements to the
   pip requirements file to build on readthedocs

kaushik94/tardis,orbitfold/tardis,orbitfold/tardis,Minhmo/tardis,Tobychev/tardis,Minhmo/tardis,Tobychev/tardis,Minhmo/tardis,kaushik94/tardis,orbitfold/tardis,kaushik94/tardis,Tobychev/tardis,kaushik94/tardis,orbitfold/tardis