Update readme

Signed-off-by: fpr <[email protected]>

Author: fpr

README.md

@@ -1,2 +1,49 @@
 # STM32RTC
 A RTC library for STM32.
+
+# API
+
+This library is based on the Arduino RTCZero library.
+The library allows to take control of the internal RTC of the STM32 boards.
+
+The following functions are not supported:
+
+* **`void standbyMode()`**: use the STM32 Low Power library instead.
+* **`uint8_t getAlarmMonth()`**: month not supported by STM32 RTC architecture.
+* **`uint8_t getAlarmYear()`**: year not supported by STM32 RTC architecture.
+* **`void setAlarmMonth(uint8_t month)`**: month not supported by STM32 RTC architecture.
+* **`void setAlarmYear(uint8_t year)`**: year not supported by STM32 RTC architecture.
+* **`void setAlarmDate(uint8_t day, uint8_t month, uint8_t year)`**: month and year not supported by STM32 RTC architecture.
+
+The following functions have been added to support specific STM32 RTC features:
+
+_RTC hours mode (12 or 24)_
+* **`void begin(RTCHourFormats_t format)`**
+
+_RTC clock source_
+* **`void setClockSource(sourceClock_t source)`** : this function must be called before `begin()`
+
+_SubSeconds management_
+* **`uint32_t getSubSeconds(void)`**
+* **`void setSubSeconds(uint32_t subSeconds)`**
+
+_Hour format (AM or PM)_
+* **`uint8_t getHours(Hour12_AM_PM_t *format)`**
+* **`void setHours(uint8_t hours, Hour12_AM_PM_t format)`**
+* **`void setTime(uint8_t hours, uint8_t minutes, uint8_t seconds, uint32_t subSeconds, Hour12_AM_PM_t format)`**
+* **`void setAlarmHours(uint8_t hours, Hour12_AM_PM_t format)`**
+* **`uint8_t getAlarmHours(Hour12_AM_PM_t *format)`**
+* **`void setAlarmTime(uint8_t hours, uint8_t minutes, uint8_t seconds, Hour12_AM_PM_t format)`**
+
+_Week day configuration_
+* **`uint8_t getWeekDay(void)`**
+* **`void setWeekDay(uint8_t weekDay)`**
+* **`void setDate(uint8_t weekDay, uint8_t day, uint8_t month, uint8_t year)`**
+
+Refer to the Arduino RTC documentation for the other functions  
+http://arduino.cc/en/Reference/RTC
+
+## Source
+
+Source files available at:  
+https://github.com/stm32duino/STM32RTC