CPack/DragNDrop: Update documentation to include new variables

Document the new behavior of `CPACK_DMG_BACKGROUND_IMAGE` and the
purpose of `CPACK_DMG_DS_STORE_SETUP_SCRIPT`.

Help/release/dev/better-looking-mac-packages.rst

@@ -0,0 +1,8 @@
+better-looking-mac-packages
+---------------------------
+
+* The :module:`CPackDMG` module learned new variable to specify AppleScript
+  file run to customize appearance of ``DragNDrop`` installer folder,
+  including background image setting using supplied PNG or multi-resolution
+  TIFF file.  See the :variable:`CPACK_DMG_DS_STORE_SETUP_SCRIPT` and
+  :variable:`CPACK_DMG_BACKGROUND_IMAGE` variables.

Modules/CPackDMG.cmake

@@ -26,15 +26,25 @@
 #  Path to a custom DS_Store file. This .DS_Store file e.g. can be used to
 #  specify the Finder window position/geometry and layout (such as hidden
 #  toolbars, placement of the icons etc.). This file has to be generated by
-#  the Finder (either manually or through OSA-script) using a normal folder
+#  the Finder (either manually or through AppleScript) using a normal folder
 #  from which the .DS_Store file can then be extracted.
 #
+# .. variable:: CPACK_DMG_DS_STORE_SETUP_SCRIPT
+#
+#  Path to a custom AppleScript file.  This AppleScript is used to generate
+#  a .DS_Store file which specifies the Finder window position/geometry and
+#  layout (such as hidden toolbars, placement of the icons etc.).
+#  By specifying a custom AppleScript there is no need to use
+#  CPACK_DMG_DS_STORE, as the .DS_Store that is generated by the AppleScript
+#  will be packaged.
+#
 # .. variable:: CPACK_DMG_BACKGROUND_IMAGE
 #
-#  Path to a background image file. This file will be used as the background
-#  for the Finder Window when the disk image is opened.  By default no
-#  background image is set. The background image is applied after applying the
-#  custom .DS_Store file.
+#  Path to an image file to be used as the background.  This file will be
+#  copied to .background/background.<ext>, where ext is the original image file
+#  extension.  The background image is installed into the image before
+#  CPACK_DMG_DS_STORE_SETUP_SCRIPT is executed or CPACK_DMG_DS_STORE is
+#  installed.  By default no background image is set.
 #
 # .. variable:: CPACK_DMG_SLA_DIR
 #