README.md: clarify purpose of ome-formats project

It is a place specifically for OME formats (i.e., those supported by
Bio-Formats), including translation to and from OME-XML.

There will certainly be other "non-core" SCIFIO formats in the future
that are not within the scope of OME and Bio-Formats; such other
non-core formats will need their own repository. In general, grouping
formats according to their associated data model(s) seems effective.