Arm backend: Add info about setup_path.sh and arm-none-eabi-gcc

Describe that you need to run setup_path.sh when starting a new shell
and that you might want to make sure the right arm-none-eabi-gcc is
used.

Signed-off-by: Zingo Andersen <zingo.andersen@arm.com>
Change-Id: Ibb9c9561005e8f6b78d7ff675e55d1b720e5a5a5