Update changelog

Author: sandeepmistry

CHANGELOG

@@ -3,6 +3,11 @@ SAMD CORE
 * Fixed Serial.flush() blocking before any writes. Thanks @hangmoh
 * Added snprintf_P to avr/pgmspace.h stub. Thanks @jantje
 * Another small fix to String iterators. Thanks @Ivan-Perez @Chris--A
+* Fixes SerialUSB.write(...) returning 0 instead of byte written
+* Added Uart::availableForWrite()
+* Added defines for RAMSTART, RAMSIZE, RAMEND
+* Fixed writing LOW to a digital input pin blocking subsequent digitalRead attempts
+* Fixed digitalWrite() unnecessarily activating the pull-up resistor
 
 SAMD CORE 1.6.6 2016.05.19