Create transmission25 ports and UPDATING entry to explain.
Sadly, Transmission has stopped supporting GTK 2.0 (it had to happen some day).
They now require GTK 3.2.0 , which we currently lack, so for those who
wish to use the GTK frontend need to stay on version 2.5
When our GTK is upgraded I'll make MOVED entries to make the transition back to
2.6 as seamless as possible.