Add new RPi installer info to readme

New RPi install script available at
http://tmrh20.github.io/RF24Installer/RPi/install.sh
Fix: RPi memory corruption

- Per #46 the read_payload() function was over-reading data into the
user provided buffer
Update readme for installer

- remove sudo from installer instructions, to prevent the cloned
dirs/files from being downloaded as root
- Add D option with example info
Merge remote-tracking branch 'TMRh20/master'
RF24Usb implementation