Add the GFDL license file

NO_CHANGELOG
2 jobs for master in 11 minutes and 40 seconds (queued for 159 minutes and 43 seconds)
Status Job ID Name Coverage
  Build
passed #95349
build_clazy_clang_tidy

00:06:50

passed #95348
linux kf5-qt5 qt5.15

00:04:49

31.0%