Autogen: Add release notes for CMAKE_AUTOGEN_VERBOSE

Help/release/dev/CMAKE_AUTOGEN_VERBOSE.rst

@@ -0,0 +1,6 @@
+CMAKE_AUTOGEN_VERBOSE
+---------------------
+
+* The new variable :variable:`CMAKE_AUTOGEN_VERBOSE` allows
+  to increase the verbosity of :prop_tgt:`AUTOMOC`, :prop_tgt:`AUTOUIC` and
+  :prop_tgt:`AUTORCC` from within CMakeLists.txt.