Add note about --unsafe-perm
Add note about --unsafe-perm

I received the following error when trying to install homebridge on a RaspberryPi. Assuming I'm not the first or the last to see this error, I thought adding a note to README about the --unsafe-perm flag would aid others.
nfarina/homebridge,snowdd1/homebridge,nfarina/homebridge,inonprince/homebridge,chrisi21/homebridge