COPYING: Add a note about the included Doxygen-generated HTML.

COPYING

@@ -21,6 +21,17 @@ XZ Utils Licensing
         XZ Utils specific documentation files in other directories
         are in the public domain.
 
+        Note: The JavaScript files (under the MIT license) have
+        been removed from the Doxygen-generated HTML version of the
+        liblzma API documentation. Doxygen itself is under the GNU GPL
+        but the remaining files generated by Doxygen are not affected
+        by the licenses used in Doxygen because Doxygen licensing has
+        the following exception:
+
+            "Documents produced by doxygen are derivative works
+            derived from the input used in their production;
+            they are not affected by this license."
+
       - Translated messages are in the public domain.
 
       - The build system contains public domain files, and files that