Add Read the Docs

We are embracing the ideas of Openness, and want to be included in your sweet directory :)

Our source code is available online: https://github.com/rtfd/readthedocs.org -- that's pretty open, right? :)

Logo: https://www.dropbox.com/s/82k1ctbt54x3v02/rtd-logo-on-white.png?dl=0