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.
Oh, I assumed readthedocs would wipe the directory. I removed the 
astropy-helpers and it still build the documentation, so I assumed it 
was not needed. I'll put it back in.

On 06/09/2014 04:04 AM, Thomas Robitaille wrote:
>
> In docs/rtd-pip-requirements:
>
> > @@ -0,0 +1,4 @@
> > +numpy
>
> 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.
>
> —
> Reply to this email directly or view it on GitHub 
> <https://github.com/astropy/package-template/pull/78/files#r13536102>.
>