Add change log for variant compliance

VARIANT_COMPLIANCE_CHANGELOG

+SAMD CORE 1.6.5
+
+* digitalPinToInterrupt #define moved to Arduino.h, variant.h must no longer define it
+
+SAMD CORE 1.6.3
+
+* Timer for pin PWM selected based on value of PIN_ATTR_TIMER_ALT or PIN_ATTR_TIMER, 
+  prior to this pin type was used