ref-manual: Added copyleft_filter class description.

(From yocto-docs rev: 2259f448cf1d102cbf06ad371e90c439812038cb)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/ref-classes.xml

@@ -488,6 +488,17 @@
     </para>
 </section>
 
+<section id='ref-classes-copyleft_filter'>
+    <title><filename>copyleft_filter.bbclass</filename></title>
+
+    <para>
+        A class used by the archiver and the
+        <link linkend='ref-classes-copyleft_compliance'><filename>copyleft_compliance</filename></link>
+        classe for filtering licenses.
+        This is an internal class and is not intended to be used directly.
+    </para>
+</section>
+
 <section id='ref-classes-core-image'>
     <title><filename>core-image.bbclass</filename></title>