DOC: Add short doc page for SelectFiles

I added a short doc page for the `SelectFiles` interface, which it seemed like we should have.

*Note:* I have not been able to build the docs on my machine, due to an error that appears unrelated but that I don't have time to track down. Here is the traceback:

```
preparing documents... done
writing output... [  1%] api/generated/nipype.caching.memory
Exception occurred:
  File "/Library/Frameworks/Python.framework/Versions/7.3/lib/python2.7/site-packages/sphinx/ext/graphviz.py", line 219, in render_dot_html
    mapfile = open(outfn + '.map', 'rb')
IOError: [Errno 2] No such file or directory: '/Users/mwaskom/Code/nipype/doc/_build/html/_images/inheritance-13dfa5eb83a19255bbe2076e8fa65c5764fa0f2c.png.map'
The full traceback has been saved in /var/folders/rr/svlz5yhs0tsdqnsp8g4qg65m0000gn/T/sphinx-err-8iHGZi.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
Either send bugs to the mailing list at <http://groups.google.com/group/sphinx-dev/>,
or report them in the tracker at <http://bitbucket.org/birkenfeld/sphinx/issues/>. Thanks!
make: *** [htmlonly] Error 1
```

If someone who does have doc-building up and running wants to double-check that my addition looks OK, that would be great. I would also accept quick solutions to my issue :).