Add descriptions for custom schema attributes

These are shown in Finder when you start a smart folder (or do a
"Find"). Click the popup labelled "Kind" and choose "Other..." The table
in the sheet that appears uses these descriptions.

epubMDI/schema.strings

@@ -6,4 +6,6 @@
   Copyright (c) 2012 Chris Ridd. All rights reserved.
 */
 org_idpf_epub_container_metadata_illustrators = "Illustrators";
-org_idpf_epub_container_metadata_translators = "Translators";
+org_idpf_epub_container_metadata_illustrators.Description = "Person or organization who illustrates a written text";
+org_idpf_epub_container_metadata_translators = "Translators";
+org_idpf_epub_container_metadata_translators.Description = "Person or organization who translates a text from one language into another";