Add default SF12 CAD parameters for SX126x

[S5NC]

Additions using values added to AN1200.48: LoRa Channel Activity Detection (CAD) with SX126x. Revision to document noted by

Revision History: Version 2.1 (November 2019) Add SF12 operation {...}

The README says the possible values for CAD_DETECT_PEAK are [22-25], but this isn't true, a much larger range works.

The optimal settings are optimised for specifically a bandwidth of 125 kHz, this isn't made clear.