DOC: Add document describing file formats

I have added a new RST document that describes the file formats Emperor uses
to display plots. Classic coordinate files (as generated by QIIME
<= 1.8.0), scikit-bio coordinates files and QIIME metadata mapping files are
all to some extent described here.

Fixes #286
Thanks for catching this @antgonza, I ended up changing the sentence as
it seemed a bit redundant.

On (Sep-15-14| 7:44), Antonio Gonzalez wrote:
> > @@ -0,0 +1,67 @@
> > +.. _formats:
> > +
> > +.. index:: formats
> > +
> > +File Formats
> > +============
> > +
> > +Emperor uses two main files as inputs, a mapping file and a coordinates file,
> > +here we will describe the format these two files should be in for Emperor to
> 
> the format *of* these two files so Emperor works correctly.
> 
> ---
> Reply to this email directly or view it on GitHub:
> https://github.com/biocore/emperor/pull/302/files#r17545596