Added STM32RTC as a requirement

I could not compile STM32LowPower without https://github.com/stm32duino/STM32RTC

Author: TTMaZa

README.md

@@ -3,6 +3,7 @@ Arduino library to support STM32 Low Power.
 
 ## Requirement
  * [Arduino_Core_STM32](https://github.com/stm32duino/Arduino_Core_STM32) version >= 1.3.0
+ * [STM32RTC](https://github.com/stm32duino/STM32RTC)
 
 ## API