generate docs for online viewing about specutils

[eteq]

Done - it will be at https://specutils.readthedocs.org/en/latest/ . Right now, due to the issues in #8, it can't build. But I've set everything up so it will build correctly on RTD once those problems are fixed.

For posterity (in case someone else needs to do this for a different affiliated package), I created a new project on the astropy RTD account, set it to this repo, added the docs/rtd-pip-requirements (see commit b9a0e9f), and then activated the github web hook for RTD to trigger automatically.