doc: Add measured boot documentation

Briefly describe the feature and specify the requirements.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index fa70292..fb043a8 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -14,6 +14,7 @@
    partitions
    cmdline
    semihosting
+   measured_boot
 
 Shell commands
 --------------