Added info for building with a pre-existing PerfCtr distribution and removed USIII warning
Added info for building with a pre-existing PerfCtr distribution and removed USIII warning

arm-hpc/papi,pyrovski/papi,arm-hpc/papi,pyrovski/papi,pyrovski/papi,pyrovski/papi,pyrovski/papi,arm-hpc/papi,arm-hpc/papi,arm-hpc/papi,pyrovski/papi,arm-hpc/papi,pyrovski/papi,arm-hpc/papi,pyrovski/papi