add note about 'dig +rrcomments'

README

@@ -89,6 +89,8 @@ BIND 9.9.0
         - DLZ modules can now retrieve the source IP address of
           the querying client.
         - 'request-ixfr' option can now be set at the per-zone level.
+        - 'dig +rrcomments' turns on comments about DNSKEY records,
+          indicating their key ID, algorithm and function
 
 BIND 9.8.0