Information on choosing a device in the ROS wrapper by USB port ID

Hi everyone,

Information has been added to the RealSense ROS wrapper about attaching to a device via a particular USB port, using an option called usb_port_id

"usb_port_id: will attach to the device with the given USB port (usb_port_id). i.e 4-1, 4-2 etc.  Default, ignore USB port when choosing a device."

https://github.com/IntelRealSense/realsense-ros#launch-parameters