docs: add Measured Boot design

Added design document for Measured Boot implementation in
TF-A.

Change-Id: I25b57ec555b289eb6bbf0a6aae014d7bf6d152fd
Signed-off-by: Manish V Badarkhe <Manish.Badarkhe@arm.com>
diff --git a/docs/design_documents/index.rst b/docs/design_documents/index.rst
index d20fc58..ecc68b2 100644
--- a/docs/design_documents/index.rst
+++ b/docs/design_documents/index.rst
@@ -11,6 +11,7 @@
    drtm_poc
    rss
    psci_osi_mode
+   measured_boot
 
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