Create citable reference for pyregion

As someone who uses pyregion quite frequently, it would be very nice if one could also cite it properly in publications. The Astrophysics Source Code Library - short ASCL - is the platform that enables that, and more: http://ascl.net/

I suggest to create an entry for pyregion in the ASCL.

Start with the ASCL site linked in the issue and review how software entries are created. Use the pyregion repository description and metadata to prepare the entry; the work is done when pyregion has a citable ASCL entry.

Written by the indexing model from the issue text.