Add bug to css animation bugs list

@keyframes are not supported in Firefox in inline or scoped stylesheets, which is against spec, but a known will-not-fix bug. See https://bugzilla.mozilla.org/show_bug.cgi?id=830056 and http://css-tricks.com/animate-to-an-inline-style/
Merge pull request #741 from glinden/patch-1

Add bug to css animation bugs list