Add links to forum

Changes to be committed:
	modified:   README.md
Add low battery voltage indicator for X4.

LEDs will start to blink while battery voltage is below threshold.
Unlike the original firmware, blinking will stop again if voltage rises
above threshold again.

LED blinking patterns:
Off during IMU calibration
Slow blink while battery voltage low
Fast blink while connection to transmitter lost.
Short blinks while not armed
On otherwise.

Voltage measurement turned out to be complicated:
ADC reference voltage is internally tied to supply voltage in Mini54.
Supply voltage is generated by LM6206 3.0V low dropout linear voltage
regulator with +/- 2% output tolerance.
Problem: even this low dropout type has max. 680mV dropout
voltage. So while flying the reference voltage might drop below 3.0V.
When the reference voltage drops together with the battery voltage, the
battery voltage cannot be measured anymore.
The internal bandgap voltage should not be influenced by this.
Unfortunately the bandgap voltage is specified with high tolerance.
Assuming that the bandgap voltage is not excatly known beforehand, but
it's at least stable, the battery voltage measurement is performed like this:

Measure initial bandgap voltage after power-on. At that time the battery is not under load
and the external reference voltage is good.
In the main loop alternatingly measure battery voltage and bandgap voltage.
Use current bandgap and initial bandgap measurements to compensate any reference
voltage changes.

On branch battery
Changes to be committed:
	new file:   lib-Mini51/hal/lib_adc.c
	new file:   lib-Mini51/hal/lib_adc.h
	modified:   src/bradwii.c
	modified:   src/bradwii.h
	modified:   src/config_X4.h
Add links to forum

Changes to be committed:
	modified:   README.md
Merge branch 'battery'
Add ADC library and battery empty detection.
Add LED patterns

Changed x4_set_leds() to be able to set state of individual LEDs.
Implement different patterns for various states: pre-binding, calibration, connection lost etc.
Update README with new features.

Changes to be committed:
	modified:   README.md
	modified:   src/bradwii.c
	modified:   src/config_X4.c
	modified:   src/config_X4.h
	modified:   src/imu.c
	modified:   src/rx_x4.c