DOC: expanding comparison with R section
- [ ] closes #9815
- [ ] tests added / passed
- [ ] passes `git diff upstream/master | flake8 --diff`
- [ ] whatsnew entry

This is the beginning of a quick reference section.  It's incomplete,
just did a rough translation of
http://nbviewer.jupyter.org/urls/gist.githubusercontent.com/TomAugspurger/6e052140eaa5fdb6e8c0/raw/811585624e843f3f80b9b6fe89e18119d7d2d73c/dplyr_pandas.ipynb
into tables.  Should try to get some R experts to comment, and it would
be nice to have the pandas versions link to docs for the functions being
used, but I'm terrible at reStructuredText and gave up for the moment.
