Update README.md

fpistm · web-flow · commit d5c9ffa6ebbf · 2018-05-28T09:38:34.000+02:00
diff --git a/README.md b/README.md
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 # STM32RTC
 A RTC library for STM32.
 
+## Requirement
+* [Arduino_Core_STM32](https://github.com/stm32duino/Arduino_Core_STM32) version >= 1.3.0
+
 # API
 
 This library is based on the Arduino RTCZero library.
