ENH: added comment for used DCMTK revision

CMakeExternals/DCMTK.cmake

@@ -24,7 +24,7 @@ if(DEFINED DCMTK_DIR AND NOT EXISTS ${DCMTK_DIR})
 endif()
 
 if(NOT DEFINED DCMTK_DIR AND NOT ${CMAKE_PROJECT_NAME}_USE_SYSTEM_${proj})
-  set(revision_tag efe1a177eac040b6d41aae0db7714cd970e12bc5)
+  set(revision_tag efe1a177eac040b6d41aae0db7714cd970e12bc5) # DCMTK-3.6.1_201610.12
   if(${proj}_REVISION_TAG)
     set(revision_tag ${${proj}_REVISION_TAG})
   endif()