Add link to documentation to README.

We now have auto-building documentation at http://cusfsim.readthedocs.org/. It would be nice to add a badge to the README linking to it. The readthedocs website itself recommends the following markdown:

``` markdown
[](http://cusfsim.readthedocs.org/en/latest/?badge=latest)
```

This can be added to the README next to the other badges.

It would also be nice to add a textual link to the documentation in the README. Something like a "Documentation" section with the text "fuller documentation is available at ...".