Add note about SPDX identifiers in license.md
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 <668e8b0c79bf1255bca237efa7e924d119546d59@arm.com>
Signed-off-by: Dan Handley <f921e9c08311ccef2e6e7757de21685f66c391dc@arm.com>

lsigithub/arm-trusted-firmware_public,achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public