README: Update PPA instructions
README: Update PPA instructions

Fixes #581

Signed-off-by: Thomas Ingleby <bb80a585db4bdbe09547a7483d1a31841fd0e0e8@intel.com>

Jon-ICS/mraa,arunlee77/mraa,spitfire88/mraa,arfoll/mraa,KurtE/mraa,yongli3/mraa,sergev/mraa,alext-mkrs/mraa,w4ilun/mraa,ProfFan/mraa,malikabhi05/mraa,intel-iot-devkit/mraa,yongli3/mraa,stefan-andritoiu/mraa,g-vidal/mraa,sergev/mraa,KurtE/mraa,ProfFan/mraa,g-vidal/mraa,intel-iot-devkit/mraa,arfoll/mraa,ncrastanaren/mraa,Propanu/mraa,KurtE/mraa,Propanu/mraa,sergev/mraa,arunlee77/mraa,arfoll/mraa,arfoll/mraa,g-vidal/mraa,arunlee77/mraa,Jon-ICS/mraa,intel-iot-devkit/mraa,ncrastanaren/mraa,alext-mkrs/mraa,jontrulson/mraa,yongli3/mraa,ncrastanaren/mraa,Jon-ICS/mraa,arfoll/mraa,malikabhi05/mraa,stefan-andritoiu/mraa-gpio-chardev,whbruce/mraa,ncrastanaren/mraa,spitfire88/mraa,yongli3/mraa,spitfire88/mraa,whbruce/mraa,g-vidal/mraa,whbruce/mraa,jontrulson/mraa,Propanu/mraa,spitfire88/mraa,jontrulson/mraa,stefan-andritoiu/mraa,stefan-andritoiu/mraa-gpio-chardev,stefan-andritoiu/mraa,Propanu/mraa,alext-mkrs/mraa,intel-iot-devkit/mraa,malikabhi05/mraa,sergev/mraa,stefan-andritoiu/mraa-gpio-chardev,w4ilun/mraa,whbruce/mraa,yongli3/mraa,intel-iot-devkit/mraa,stefan-andritoiu/mraa,jontrulson/mraa,stefan-andritoiu/mraa-gpio-chardev,Propanu/mraa,KurtE/mraa,alext-mkrs/mraa,w4ilun/mraa,KurtE/mraa,jontrulson/mraa,ProfFan/mraa,stefan-andritoiu/mraa,alext-mkrs/mraa,ProfFan/mraa,w4ilun/mraa,arunlee77/mraa,g-vidal/mraa,ncrastanaren/mraa,malikabhi05/mraa,malikabhi05/mraa,sergev/mraa,stefan-andritoiu/mraa-gpio-chardev