There was some confusion what was needed to utilize the slave support,
so let's be more precise about this. Add an introductory paragraph to
the development section while we are here.
Signed-off-by: NWolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: NWolfram Sang <wsa@the-dreams.de>