Docs: Add security handling

This is a landing page for TF-M security, including the list of
advisories and how the security issue disclosures or reports.

Change-Id: Ibe7918876af0537ea170f38022b6eb19cc849212
Signed-off-by: Karl Zhang <karl.zhang@arm.com>
diff --git a/docs/reference/index.rst b/docs/reference/index.rst
index bb9cf6a..7e81349 100644
--- a/docs/reference/index.rst
+++ b/docs/reference/index.rst
@@ -7,6 +7,7 @@
 
     changelog
     glossary
+    security
     */index
     /tools/index