Add documentation of new command

CMakeLists.txt

@@ -25,6 +25,8 @@ include(DuneMacros)
 
 # start a dune project with information from dune.module
 dune_project()
+
+# create library target and export it as Dune::ISTL
 dune_add_library(duneistl INTERFACE
   EXPORT_NAME ISTL
   LINK_LIBRARIES Dune::Common)