Add docs about DCO
Add docs about DCO

Create a SubmittingPatches.md file similar to the one in Linux.  This
includes the Developer Certificate of Origin which clarifies the meaning
of the Signed-off-by trailer in the message.

Patches merged after this, should have Signed-off-by lines.

Signed-off-by: David Brown <0af22db655f3ade5845314d695c1f4910314b0cb@linaro.org>

tamban01/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,runtimeco/mcuboot,utzig/mcuboot,runtimeco/mcuboot,utzig/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,utzig/mcuboot,utzig/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,utzig/mcuboot