add bug notes about dragndrop on mobile

"fixes" #637

DnD has worked in iOS since v 6 ([demo](http://harlancampbelljr.com/test4art/story_html5.html))

Blink has [this bug](http://crbug.com/156390), and I [filed one for FFOS](https://bugzilla.mozilla.org/show_bug.cgi?id=945073). No idea how to submit a bug on IE Mobile, but I reached out to @reybango to find out