binman: Add more documentation about binman usage

This is an attempt to answer the comments provided by Xavier [1].

[1] https://lore.kernel.org/all/Yulcol7HpTHtjXTX@begut/



Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>