Add CMakeLists.txt with ATLAS macros.

CMakeLists.txt

+# The name of the package
+ATLAS_SUBDIR(JetSelectionHelper)
+
+# External Dependencies
+FIND_PACKAGE( ROOT )
+
+# Add binary
+ATLAS_ADD_LIBRARY ( JetSelectionHelperLib JetSelectionHelper/JetSelectionHelper.h src/JetSelectionHelper.cxx
+		  PUBLIC_HEADERS JetSelectionHelper
+		     INCLUDE_DIRS ${ROOT_INCLUDE_DIRS}
+		     LINK_LIBRARIES ${ROOT_LIBRARIES}
+		     		     xAODEventInfo
+				     xAODRootAccess
+				     xAODJet)