Documented a bit better yesterday's changes

CMakeLists.txt

@@ -6,6 +6,7 @@ set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} ${CMAKE_SOURCE_DIR}/cmake)
 #---Options-------------------------------------------------------------------------
 option(DD4HEP_USE_XERCESC "Enable 'Detector Builders' based on XercesC" OFF)
 option(DD4HEP_USE_PYROOT "Enable 'Detector Builders' based on PyROOT" ON)
+option(DD3HEP_WITH_GEANT4 "Enable the simulation part based on Geant4" ON)
 
 find_package(ROOT REQUIRED)
 if(DD4HEP_USE_XERCESC)
@@ -14,15 +15,18 @@ endif()
 
 #---Packages------------------------------------------------------------------------
 add_subdirectory(DDCore)
-add_subdirectory(DDGeant4)
+if(DD3HEP_WITH_GEANT4)
+  add_subdirectory(DDGeant4)
+endif()
 
+#---Examples------------------------------------------------------------------------
 add_subdirectory(DDExamples/ILDExDet)
 add_subdirectory(DDExamples/CLICSiD)
-
 add_subdirectory(DDExamples/ILDExDisplay)
 add_subdirectory(DDExamples/ILDExReco)
-add_subdirectory(DDExamples/ILDExSimu)
-
+if(DD3HEP_WITH_GEANT4)
+  add_subdirectory(DDExamples/ILDExSimu)
+endif()
 
 #---Configuration-------------------------------------------------------------------
 configure_file(cmake/thisdd4hep.csh thisdd4hep.csh @ONLY)

ReadMe.txt

@@ -20,6 +20,14 @@ on the C++ XercesC parser, and another one based on Python and using the PyROOT
      DD4HEP_USE_PYROOT:BOOL
      DD4HEP_USE_XERCESC:BOOL
 
+- DDGeant4
+
+  Is the package that contains the conversion of DD4hep geometry into Geant4 geometry to be used for 
+simulation. The conversion is currently based on the Virtual Geometry Modeler (VGM) package that is
+build internally when required.
+  The option DD4HEP_WITH_GEANT4:BOOL controls the building or not of this package that has the dependency
+to Geant4. The Geant4 installation needs to be located using the variable -DGeant4_DIR=<G4 installation>
+
 - DDExamples
 
   Contains different detector examples (CLICSiD, ILDEx) together with example applications to illustrate