Add README.md for DISCO_L072CZ_RWAN1 describing restrictions

Signed-off-by: Kristian Nielsen <[email protected]>

Author: knielsen

variants/DISCO_L072CZ_LRWAN1/README.md

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+# Discovery L072CZ-LRWAN1 support
+## Board [B-L072Z-LRWAN1](http://www.st.com/en/evaluation-tools/b-l072z-lrwan1.html)
+## MCU [STM32L072CZ](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.
+
+There is no PWM on Arduino `D5` (`PB7`), `D6` (`PB2`) and `D9` (`PB12`) unlike Arduino Uno.
+
+For further information, please refers to the [UM2115: Discovery kit for LoRaWAN (TM) and LPWAN protocols with STM32L0](http://www.st.com/resource/en/user_manual/dm00329995.pdf)