adds README

Author: toddtreece

README.md

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+# Adafruit Package Lists for the Arduino v1.6.4+ Board Manager
+
+This repo contains the custom `package_*_index.json` files that can be used to add new
+third party boards to the Arduino v1.6.4+ IDE.
+
+## List of 3rd Party Boards
+
+https://github.com/arduino/Arduino/wiki/Unofficial-list-of-3rd-party-boards-support-urls