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 used astropy-helpers and it worked. Then I removed it, and it worked, too.
I don't quite understand why (I am not that deep in the astropy-helpers 
process), but I assume it's because astro-py helpers is installed anyway 
when astropy is installed.

Stable version: The advantage of doing it this way is that we never need 
to to update the template and affiliated packages never need to update 
the rdt-pip-requirements file.
Since we generally make sure that master passes all tests, I thought 
using the latest version (not the latest stable) is OK for this purpose. 
Also, when someone applies the package template, he/she is probably 
relatively early in the process of coding the affiliated package and 
does not worry yet about back-wards compatibility etc.
On the other hand, I see that we might not want to use a version that 
changes daily. If I write "astropy" I think that should use the latest 
released (stable) version, right?