doc: describe the md command

Provide a man-page for the md command.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 09372d4..f7b706f 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -28,6 +28,7 @@
    load
    loady
    mbr
+   md
    pstore
    qfw
    sbi