Add 3.5" floppy disk as a medium format

I've tagged some releases already - https://musicbrainz.org/tag/format-floppy%20disk/release
There are more releases on http://www.discogs.com/search/?q=&format_exact=Floppy+Disk&format_exact=3.5%22

We might also want 5.25" floppy disk at some point, Discogs does have a few ( http://www.discogs.com/search/?q=&format_exact=Floppy+Disk&format_exact=5.25%22 ), but none of the ones I've tagged so far are that type, so I've limited this to just the more common 3.5" type.