doc: document echo command

Provide a man-page for the echo command.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/usage/index.rst b/doc/usage/index.rst
index 6f49652..317db3b 100644
--- a/doc/usage/index.rst
+++ b/doc/usage/index.rst
@@ -17,6 +17,7 @@
    bootefi
    bootmenu
    button
+   echo
    exit
    for
    mbr