update the main .gitignore file (Settings.cmake, build*)
update the main .gitignore file (Settings.cmake, build*)

- prepare the usage of a local (i.e. not under version control) settings file
  by excluding it from the version control

- exclude the usual choice for a CMake based out-of-tree build directory

SixTrack/SixTrackLib,SixTrack/SixTrackLib,SixTrack/SixTrackLib,SixTrack/SixTrackLib