clarify location of LICENSE.txt file

.gitlab-ci.yml

CMakeLists.txt

Common/CMakeLists.txt

Common/Colors.h

Common/CommandLine.cpp

Common/CommandLine.h

Common/Context.cpp

Common/Context.h

Common/CreateFile.cpp

Common/CreateFile.h

Common/DefaultOptions.h

Common/GamePad.cpp

Common/GamePad.h

Common/GmshConfig.h.in

Common/GmshDefines.h

Common/GmshGlobal.cpp

Common/GmshGlobal.h

Common/GmshMessage.cpp

Common/GmshMessage.h

Common/GmshRemote.cpp