Highlight the challenges of extending meta-schemas

This addresses the original question posed in #86, which
noted that simply saying that meta-schemas can be extended
with "allOf" is misleading and likely to lead to errors.

Author: handrews

jsonschema-core.xml

@@ -356,6 +356,11 @@
                     This extended meta-schema SHOULD be referenced using the "$schema" keyword, to
                     allow tools to follow the correct behaviour.
                 </t>
+                <t>
+                    Note that the recursive nature of meta-schemas requires re-definining
+                    recursive keywords in the extended meta-schema, as can be seen in
+                    ith the JSON Hyper-Schema meta-schema.
+                </t>
             </section>
 
         </section>