[U-Boot] [PATCH 12/13] binman: Add documentation for the symbol feature

[sjg at google.com]

Hi Simon,

> Add this feature to the README.
>
> Signed-off-by: Simon Glass <sjg at chromium.org>
> ---
>
>  tools/binman/README | 32 +++++++++++++++++++++++++++++++-
>  1 file changed, 31 insertions(+), 1 deletion(-)
>
Applied to u-boot-dm thanks!