Release notes: Add release notes for ADDITIONAL_CLEAN_FILES properties

This adds release notes for the `ADDITIONAL_CLEAN_FILES` target and directory
properties.  It also adds a deprecation release note for the
`ADDITIONAL_MAKE_CLEAN_FILES` directory property.

Help/release/dev/additional_clean_files.rst

@@ -0,0 +1,10 @@
+additional_clean_files
+----------------------
+
+* New target property :prop_tgt:`ADDITIONAL_CLEAN_FILES` and directory property
+  :prop_dir:`ADDITIONAL_CLEAN_FILES` were added.  They allow to register
+  additional files that should be removed during the clean stage.
+
+* Directory property :prop_dir:`ADDITIONAL_MAKE_CLEAN_FILES` was marked
+  deprecated.  The new directory property :prop_dir:`ADDITIONAL_CLEAN_FILES`
+  should be used instead.