Change README to specify build type

Previously the documentation implied that a release build would be
created by default. The default is toolchain-specific, hence the
recommendation in the CMake documentation:
"Users should always explicitly specify the build type"

Signed-off-by: Michael Platings <michael.platings@arm.com>
Change-Id: I0b5b793600a548f7b7dd540d588f212f78aae5af