README.md: update mraa instructions
README.md: update mraa instructions

Signed-off-by: Brendan Le Foll <3e0e8c4069934e8b47d99704927a85c66c0ceb33@intel.com>

KurtE/mraa,arunlee77/mraa,arfoll/mraa,stefan-andritoiu/mraa-gpio-chardev,malikabhi05/mraa,stefan-andritoiu/mraa-gpio-chardev,alext-mkrs/mraa,whbruce/mraa,whbruce/mraa,Jon-ICS/mraa,stefan-andritoiu/mraa,ProfFan/mraa,intel-iot-devkit/mraa,Propanu/mraa,KurtE/mraa,stefan-andritoiu/mraa,sergev/mraa,arunlee77/mraa,KurtE/mraa,andreivasiliu2211/mraa,ncrastanaren/mraa,ncrastanaren/mraa,Jon-ICS/mraa,jontrulson/mraa,Jon-ICS/mraa,arfoll/mraa,spitfire88/mraa,jontrulson/mraa,spitfire88/mraa,w4ilun/mraa,yongli3/mraa,alext-mkrs/mraa,arfoll/mraa,intel-iot-devkit/mraa,g-vidal/mraa,Propanu/mraa,andreivasiliu2211/mraa,w4ilun/mraa,w4ilun/mraa,ProfFan/mraa,Propanu/mraa,jontrulson/mraa,sergev/mraa,w4ilun/mraa,alext-mkrs/mraa,intel-iot-devkit/mraa,arunlee77/mraa,g-vidal/mraa,malikabhi05/mraa,sergev/mraa,g-vidal/mraa,ncrastanaren/mraa,yongli3/mraa,Propanu/mraa,alext-mkrs/mraa,ProfFan/mraa,yongli3/mraa,intel-iot-devkit/mraa,whbruce/mraa,jontrulson/mraa,alext-mkrs/mraa,Propanu/mraa,yongli3/mraa,g-vidal/mraa,whbruce/mraa,stefan-andritoiu/mraa,g-vidal/mraa,arfoll/mraa,andreivasiliu2211/mraa,stefan-andritoiu/mraa-gpio-chardev,stefan-andritoiu/mraa,arunlee77/mraa,stefan-andritoiu/mraa-gpio-chardev,ncrastanaren/mraa,yongli3/mraa,malikabhi05/mraa,stefan-andritoiu/mraa-gpio-chardev,stefan-andritoiu/mraa,spitfire88/mraa,sergev/mraa,KurtE/mraa,arfoll/mraa,KurtE/mraa,intel-iot-devkit/mraa,spitfire88/mraa,andreivasiliu2211/mraa,ncrastanaren/mraa,malikabhi05/mraa,jontrulson/mraa,malikabhi05/mraa,ProfFan/mraa,sergev/mraa