Ignore platform.local.txt

This is a file that can be used to tweak platform.txt, once
arduino/Arduino#1985 is merged.

Author: matthijskooijman

.gitignore

@@ -16,3 +16,4 @@
 /atmega16u2/lufa-100807/Projects/arduino-usbserial/Arduino-usbserial.hex
 /atmega16u2/lufa-100807/Projects/arduino-usbserial/Arduino-usbserial.s
 /atmega16u2/lufa-100807/Projects/arduino-usbserial/pinoccio-16U2-combined.hex
+/avr/platform.local.txt