docs(threat model): add TF-A threat model

This is the first release of the public Trusted
Firmware A class threat model. This release
provides the baseline for future updates to be
applied as required by developments to the
TF-A code base.

Signed-off-by: Zelalem Aweke <zelalem.aweke@arm.com>
Change-Id: I3c9aadc46196837679f0b1377bec9ed4fc42ff11
diff --git a/docs/index.rst b/docs/index.rst
index cb53127..3760855 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -15,6 +15,7 @@
    perf/index
    security_advisories/index
    design_documents/index
+   threat_model/index
    change-log
    change-log-upcoming
    glossary
@@ -83,7 +84,7 @@
 
 --------------
 
-*Copyright (c) 2013-2020, Arm Limited and Contributors. All rights reserved.*
+*Copyright (c) 2013-2021, Arm Limited and Contributors. All rights reserved.*
 
 .. _Armv7-A and Armv8-A: https://developer.arm.com/products/architecture/a-profile
 .. _Secure Monitor: http://www.arm.com/products/processors/technologies/trustzone/tee-smc.php