[designspaceLib] Document proposed public.skipExportGlyphs lib key (#1534)

* Document proposed public.skipExportGlyphs lib key

Doc/source/designspaceLib/readme.rst

@@ -1044,9 +1044,24 @@ Recommendation for editors
 -  Use ``documentObject.updateFilenameFromPath()`` to explicitly set the
    **filename** attributes for all instance and source descriptors.
 
-.. 7-this-document:
+.. 7-common-lib-key-registry:
 
-7 This document
+7 Common Lib Key Registry
+=========================
+
+public.skipExportGlyphs
+-----------------------
+
+This lib key works the same as the UFO lib key with the same name. The
+difference is that applications using a Designspace as the corner stone of the
+font compilation process should use the lib key in that Designspace instead of
+any of the UFOs. If the lib key is empty or not present in the Designspace, all
+glyphs should be exported, regardless of what the same lib key in any of the
+UFOs says.
+
+.. 8-this-document:
+
+8 This document
 ===============
 
 -  The package is rather new and changes are to be expected.