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/threat_model/index.rst b/docs/threat_model/index.rst
new file mode 100644
index 0000000..e8f09b9
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+Threat Model
+=============
+
+.. toctree::
+   :maxdepth: 1
+   :caption: Contents
+   :numbered:
+
+   threat_model
+
+--------------
+
+*Copyright (c) 2021, Arm Limited and Contributors. All rights reserved.*