[Gimp-user] put something on webpage about starting gimp

[patrick <patrick eldwick org uk>]

It is far from clear how to start gimp after using the flatpak download 
system. Maybe it will sort itself out after rebooting the computer but 
the flatpak install is rather convoluted (compared with the rather 
simple and very quick download of blender, say, as a compressed folder). 
And after it's complete there is no indication on the GIMP site of how 
to start it running. The old version 2.8 (installed by the ubuntu 
system) still runs when clicking on the icon. A search on line 
eventually finds other people who have run into this problem and the 
solution.