feat(measured boot): add documentation to build and run PoC

Add documentation to build and run a PoC based on the OP-TEE toolkit
to show how TF-A Measured Boot can interact with a third party (f)TPM
service.

Signed-off-by: Javier Almansa Sobrino <javier.almansasobrino@arm.com>
Change-Id: I11ac99c4ff54ea52aba0731aa7f707d7cd0c4216
diff --git a/docs/design_documents/index.rst b/docs/design_documents/index.rst
index 187510a..c82d2ee 100644
--- a/docs/design_documents/index.rst
+++ b/docs/design_documents/index.rst
@@ -7,6 +7,7 @@
    :numbered:
 
    cmake_framework
+   measured_boot_poc
 
 --------------