Example sketch to send battery voltage every x seconds with nRF5x

[probonopd]

Please add an example sketch to send battery voltage every x seconds with nRF5x.

The sketch should be optimized for low power consumption to run off a coin cell for a long time.

I think this kind of example will be very useful for anyone who wants to make a sensor that sends data in intervals from battery-operated sensor devices.

[probonopd]

I realize that I should have asked for this in the sister project, https://github.com/sandeepmistry/arduino-BLEPeripheral. Opened sandeepmistry/arduino-BLEPeripheral#215.