Updated install documentation

The original install documentation calls to install the new hardware files into the Arduino application directory. Under OSX this will cause the hardware files to be removed when the Arduino application is updated. The user's Sketch directory should be used to store hardware files so that they are preserved across updates.

The patch updates the documentation to describe installation into the user's Sketch directory rather than the Arduino application directory. 

The creation of the second RFduino directory inside the hardware directory is very important, otherwise the hardware files will not be scanned.