Document that gtk3 support is experimental

Change-Id: I87df8cb6a4e4e374c5c9782e695fcdf020d5707f
Reviewed-on: https://gerrit.libreoffice.org/12121
Reviewed-by: Noel Grandin <noelgrandin@gmail.com>
Tested-by: Noel Grandin <noelgrandin@gmail.com>