Update README.md

Author: fpistm

README.md

@@ -1,5 +1,9 @@
 # STM32LowPower
-Arduino library to support Low Power
+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
 
@@ -26,20 +30,25 @@ Arduino library to support Low Power
 **param** serial: pointer to a UART  
 **param** callback: pointer to a callback to call when the board is waked up.  
 
-* **`void enableWakeupFrom(TwoWire *wire, voidFuncPtrVoid callback)`**:
-enable an I2C peripheral in low power mode. See board documentation for low power mode compatibility.  
-**param** wire: pointer to I2C  
-**param** callback: pointer to a callback to call when the board is waked up.  
-
 * **`void enableWakeupFrom(STM32RTC *rtc, voidFuncPtr callback)`**
 attach a callback to the RTC peripheral.  
 **param** rtc: pointer to RTC  
 **param** callback: pointer to a callback to call when the board is waked up.  
 
+* **`void enableWakeupFrom(TwoWire *wire, voidFuncPtrVoid callback)`**:
+enable an I2C peripheral in low power mode. See board documentation for low power mode compatibility. **Currently not available.**  
+**param** wire: pointer to I2C  
+**param** callback: pointer to a callback to call when the board is waked up.  
+
+
 `Begin()` function must be called at least once before `idle()`, `sleep()`, `deepSleep()` or `shutdown()` functions.  
 
 `attachInterruptWakeup()` or `enableWakeupFrom()` functions should be called before `idle()`, `sleep()`, `deepSleep()` or `shutdown()` functions.  
 
+HardwareSerial used as Wakeup source will configure it to use HSI clock source even if another peripheral clock is configured.
+
+RTC used as Wakeup source will configure it to use LSE clock source even if another RTC clock source is selected.
+
 The board will restart when exit the deepSleep or shutdown mode.  
 
 ## Hardware state