Include notes about required files in proto_path

Not sure if this is just standard for most people, but personally I thought the library just wasn't working because I didn't have my proto_path set up right.  Props to this [Stack Overflow question on importing descriptor.proto](http://stackoverflow.com/questions/20069295/importing-google-protobuf-descriptor-proto-in-java-protocol-buffers) for helping me to see what needed to happen.