Add certificate specification

Imported from the ndn-cxx repo

Change-Id: I96040f854b16219f3be956c9aab9e13568cca900
diff --git a/changelog.rst b/changelog.rst
index 1842f7c..7c8f197 100644
--- a/changelog.rst
+++ b/changelog.rst
@@ -8,6 +8,8 @@
 
 - Require shortest encoding for TLV-TYPE and TLV-LENGTH numbers
 
+- Add ``Certificate`` specification
+
 - **Name**
 
   + Lift restriction on name component types, allowing types in the range [1, 65535]