doc: certificate format

Change-Id: I9d6427ed5e5fab159b5f96fab8b977b4371f26c3
Refs: #2861
diff --git a/docs/tutorials.rst b/docs/tutorials.rst
index 5a6f175..d4bc089 100644
--- a/docs/tutorials.rst
+++ b/docs/tutorials.rst
@@ -8,3 +8,4 @@
    tutorials/utils-ndn-regex
    tutorials/security-validator-config
    tutorials/signed-interest
+   tutorials/certificate-format