Autogen: Add release notes for Q_PLUGIN_METADATA support

Help/release/dev/Autogen_json.rst

+AutoGen json
+------------
+
+* When using :prop_tgt:`AUTOMOC`, CMake scans for the presence of the
+  ``Q_PLUGIN_METADATA`` macro and reruns moc when the file from the
+  macro's ``FILE`` argument changes.
+
+* When :prop_tgt:`AUTOMOC` detects an include statement of the form
+  ``#include "moc_<basename>.cpp"`` the respective header file is searched
+  for in the :prop_tgt:`INCLUDE_DIRECTORIES` of the target as well.