doc: Convert command-line info to rST

Take this part of the README and put it into rST format.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 719b2c9..356f2a5 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -9,6 +9,7 @@
    fit
    netconsole
    partitions
+   cmdline
 
 Shell commands
 --------------