[PATCH v2 1/5] binman: add documentation for binman sign option

[Simon Glass]

Add the documentation about binman sign option and providing an
example.

Signed-off-by: Ivan Mikhaylov <fr0st61te at gmail.com>
---
 tools/binman/binman.rst | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

Applied to u-boot-dm/next, thanks!