Add readme.txt for DISCO_L072CZ_RWAN1 describing restrictions

Signed-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>

Author: knielsen

variants/DISCO_L072CZ_LRWAN1/readme.txt

@@ -0,0 +1,29 @@
+Discovery L072CZ-LRWAN1 support.
+
+Board: http://www.st.com/en/evaluation-tools/b-l072z-lrwan1.html
+MCU: http://www.st.com/content/st_com/en/products/microcontrollers/stm32-32-bit-arm-cortex-mcus/stm32-ultra-low-power-mcus/stm32l0-series/stm32l0x2/stm32l072cz.html
+
+Restrictions:
+
+Some of the STM32L072CZY6 pins are used by the LoRa module, so fewer
+GPIOs are available on this board.
+
+These pins are not available on the board: PA6-7,15; PB0-1,3-4,10-11; PC0,13-15
+
+These pins are available, but dedicated to the LoRa module: PA1, PC1-2
+
+By default, A1, A3, A4, and A5 are not connected pins.
+
+A1 and A3 can be connected to the same GPIOs as A0 and A2 by closing
+solder bridges SB7 and SB8.
+
+A4 and A5 can be connected to PB9 and PB8 using solder bridges SB11
+and SB12 (note that PB9 and PB8 do not have any analog/ADC
+capabilities though).
+
+A third analog/ADC input is available as A6 on the PA5 pin.
+
+SPI MOSI and MISO are available on digital pins D11 and D12, but the
+SCK pin is D3 (unlike Arduino Uno where it is on D13).
+
+There is no PWM on Arduino D5 (PB7), D6 (PB2) and D9 (PB12) unlike Arduino Uno.