Autogen: Add release notes for per-config include dir

Help/release/dev/autogen-configs.rst

@@ -0,0 +1,8 @@
+autogen-configs
+---------------
+
+* When using :prop_tgt:`AUTOMOC` or :prop_tgt:`AUTOUIC` with a
+  multi configuration generator (e.g. :generator:`Xcode`),
+  included ``*.moc``,  ``moc_*.cpp`` and ``ui_*.h`` files are generated in
+  ``<AUTOGEN_BUILD_DIR>/include_<CONFIG>`` instead of
+  ``<AUTOGEN_BUILD_DIR>/include``.