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.
When you say 'it worked' - how did you test it out? Note that if you remove it then readthedocs does not wipe the environment before the next build, so removing it from the list does not uninstall. From experience with other packages, astropy_helpers needs to be included here for RTD to work correctly.