Added getResetReason

Author: hallard

doc/changes.md

@@ -6,6 +6,7 @@ title: Change Log
 
 ### Core
 
+- Add function to know last reset resaon.
 - Allow control of enabling debug and debug level from IDE
 - Make HardwareSerial::begin() and end() interrupt safe
 - Put HardwareSerial and cbuf methods called from interrupt context in RAM

doc/libraries.md

@@ -83,6 +83,8 @@ APIs related to deep sleep and watchdog timer are available in the `ESP` object,
 
 `ESP.restart()` restarts the CPU.
 
+`ESP.getResetReason()` returns String containing the last reset resaon in human readable format.
+
 `ESP.getFreeHeap()` returns the free heap size.
 
 `ESP.getChipId()` returns the ESP8266 chip ID as a 32-bit integer.