Add certificate specification

Imported from the ndn-cxx repo

Change-Id: I96040f854b16219f3be956c9aab9e13568cca900
diff --git a/index.rst b/index.rst
index 3dff38e..b72608c 100644
--- a/index.rst
+++ b/index.rst
@@ -12,5 +12,6 @@
    signature
    signed-interest
    link
+   certificate
    types
    changelog