Add certificate specification

Imported from the ndn-cxx repo

Change-Id: I96040f854b16219f3be956c9aab9e13568cca900
diff --git a/data.rst b/data.rst
index 16bdabf..50b224c 100644
--- a/data.rst
+++ b/data.rst
@@ -52,7 +52,7 @@
 +-----------------+-----------------+--------------------------------------------------------------+
 | LINK            | 1               | list of delegations (see :ref:`Link`)                        |
 +-----------------+-----------------+--------------------------------------------------------------+
-| KEY             | 2               | public key                                                   |
+| KEY             | 2               | public key (see :ref:`Certificate`)                          |
 +-----------------+-----------------+--------------------------------------------------------------+
 | NACK            | 3               | application-level NACK                                       |
 +-----------------+-----------------+--------------------------------------------------------------+