README: this is the ir2.1 branch
Mention that this is the ir2.1 branch of the scripts, to be used for IR 2.1
certifications.
Signed-off-by: 
Vincent Stehlé <vincent.stehle@arm.com>
Mention that this is the ir2.1 branch of the scripts, to be used for IR 2.1
certifications.
Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>