example file for readthedocs

Most affiliated packages will probably use readthedocs. Maybe we should have a minimal 'rtd-pip-requirements' file with some comments about how to set up readthedocs (build in virtualenv, ...) ?

This is an issue and not a PR because I not quite sure what the minimal requirements are. Probably something like:
```
-e git+http://github.com/astropy/astropy-helpers.git#egg=astropy_helpers
-e git+http://github.com/astropy/astropy.git#egg=astropy
```

EDIT: Now it is a PR.
I think you should add astropy_helpers here. Also, I think we should probably include a stable version of astropy rather than the developer version, by default?