Update readme with sample metrics

[wtip]

When looking up exporters it's nice to know what kind of metrics they provide without having to go digging in the source code.

Not that it was that hard in this case https://github.com/mdlayher/apcupsd_exporter/blob/master/upscollector.go#L23

	UPSLoadPercent                      *prometheus.Desc
	BatteryChargePercent                *prometheus.Desc
	LineVolts                           *prometheus.Desc
	LineNominalVolts                    *prometheus.Desc
	BatteryVolts                        *prometheus.Desc
	BatteryNominalVolts                 *prometheus.Desc
	BatteryNumberTransfersTotal         *prometheus.Desc
	BatteryTimeLeftSeconds              *prometheus.Desc
	BatteryTimeOnSeconds                *prometheus.Desc
	BatteryCumulativeTimeOnSecondsTotal *prometheus.Desc
	LastTransferOnBatteryTimeSeconds    *prometheus.Desc
	LastTransferOffBatteryTimeSeconds   *prometheus.Desc
	LastSelftestTimeSeconds             *prometheus.Desc
	NominalPowerWatts                   *prometheus.Desc