docs(threat model): add Threat Model for RMM

Signed-off-by: Javier Almansa Sobrino <javier.almansasobrino@arm.com>
Change-Id: I64b16d55b109c915a7820277ec14dbf290b32647
diff --git a/docs/index.rst b/docs/index.rst
index ec6bb6e..b7a0415 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -35,6 +35,13 @@
             </a>
             <p>Design documentation for RMM.</p>
         </li>
+        <li class="grid-item">
+            <a href="security/index.html">
+               <img alt="" src="_static/images/rmm-documentation.png"/>
+               <h2>Security</h2>
+            </a>
+            <p>Security.</p>
+        </li>
     </ul>
 
 .. toctree::
@@ -46,4 +53,5 @@
    getting_started/index
    process/index
    design/index
+   security/index
    glossary