minor improvements.

move docs out of script into README.md.

fix some incorrect +2kHz frequencies in $mhz_to_band hash.

switch to displaying unneeded if no entities needed for a band;
don't display if entities remain.

improve regex to allow positive dB replies (e.g., 'R+01')

add grid to potential QSO line for easy cut & paste when someone
forgets to send their grid.