Docs: Add a generic threat model of TF-M

Add a generic threat model document of TF-M.
Create a dedicated folder under docs to contain threat models.

Change-Id: Ifb332d20e1667082d054959bb7d890eda5ce025d
Signed-off-by: David Hu <david.hu@arm.com>
diff --git a/docs/threat_models/index.rst b/docs/threat_models/index.rst
new file mode 100644
index 0000000..227d4d4
--- /dev/null
+++ b/docs/threat_models/index.rst
@@ -0,0 +1,12 @@
+Threat Models
+=============
+
+.. toctree::
+    :maxdepth: 1
+    :glob:
+
+    *
+
+--------------
+
+*Copyright (c) 2020, Arm Limited. All rights reserved.*