docs: revise document of Merkle tree

Change-Id: I948dd0fb85d31d5fa92b6859bcfb41a697611021
diff --git a/docs/index.rst b/docs/index.rst
index 8dbcefa..bc31c03 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -16,6 +16,7 @@
    README
    INSTALL
    manpages
+   design
 
 - :doc:`README`
 
@@ -23,6 +24,8 @@
 
 - :doc:`manpages`
 
+- :doc:`design`
+
 Documentation for ndn-cxx developers and contributors
 +++++++++++++++++++++++++++++++++++++++++++++++++++++