docs: revise document of Merkle tree

Change-Id: I948dd0fb85d31d5fa92b6859bcfb41a697611021
diff --git a/docs/design.rst b/docs/design.rst
new file mode 100644
index 0000000..dd11bb8
--- /dev/null
+++ b/docs/design.rst
@@ -0,0 +1,8 @@
+.. _design:
+
+NSL Design Document
+==============
+
+.. toctree::
+   design-doc/merkle
+   :maxdepth: 1