Update readme.md for v1.3
Update readme.md for v1.3

Update the release notes (readme.md) for the ARM Trusted Firmware
v1.3 release.

Change-Id: Ia1f4eb1897e63eeab7d69a593ba0ad91d50043f5
Signed-off-by: Dan Handley <f921e9c08311ccef2e6e7757de21685f66c391dc@arm.com>

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