docs(drtm): steps to run DRTM implementation

Documented steps to run DRTM implementation.

Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
Change-Id: I30dd8c1088a54a8906389c2584d922862610dae0
diff --git a/docs/design_documents/index.rst b/docs/design_documents/index.rst
index 257a510..765efe6 100644
--- a/docs/design_documents/index.rst
+++ b/docs/design_documents/index.rst
@@ -9,7 +9,8 @@
    cmake_framework
    context_mgmt_rework
    measured_boot_poc
+   drtm_poc
 
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-*Copyright (c) 2020, Arm Limited and Contributors. All rights reserved.*
+*Copyright (c) 2020-2022, Arm Limited and Contributors. All rights reserved.*