docs(psci): add design proposal for OS-initiated mode

Change-Id: Ia3662e08d98d01a93951309835816969e1602624
Signed-off-by: Wing Li <wingers@google.com>
diff --git a/docs/design_documents/index.rst b/docs/design_documents/index.rst
index 3d82e69..d20fc58 100644
--- a/docs/design_documents/index.rst
+++ b/docs/design_documents/index.rst
@@ -10,6 +10,7 @@
    measured_boot_poc
    drtm_poc
    rss
+   psci_osi_mode
 
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