Updates to CHANGELOG

CHANGELOG

-SAMD CORE 1.6.16 2017.07.13
+SAMD CORE 1.6.16 2017.08.23
 
 * Is now possible to use PWM on D7 (though it can't be used with PWM on D12 at the same time). Thanks @Adminius
 * PWMs now can perform real 16-bit resolution if analogWriteResolution(16) is set. Thanks @Adminius
@@ -6,6 +6,9 @@ SAMD CORE 1.6.16 2017.07.13
 * Serial (UART) tx is now buffered.
 * Improved CRYSTALLESS stability. Thanks @tannewt
 * Some fixes to Adafruit Circuit Playground Express variant. Thanks @ladyada
+* Updated Stream and Print class
+* Native USB now supports USB Serial Number
+* Fixed pgm_read_ptr compatibility macro. Thanks @nkrkv
 
 SAMD CORE 1.6.15 2017.04.27