Add note about SPDX identifiers in license.md

Added note regarding use of SPDX identifiers following this example:
https://github.com/pocoproject/poco/blob/develop/LICENSE



Change-Id: I22a280bce57f9145e4786c5ad32f663c2c9c6545
Signed-off-by: Jilayne Lovejoy <jilayne.lovejoy@arm.com>
Signed-off-by: Dan Handley <dan.handley@arm.com>