License information is missing

Please ad some license information for this repository.

As this code is split off originally from the arduino source the license and copyright information are the same as for the arduino tree I assume.

Start by reviewing the repository for existing license or copyright files and compare the Arduino source tree's licensing information, as suggested in the issue. Confirm which license and copyright notices apply to ArduinoCore-avr, then add the missing repository information and verify that the notices are consistent.

Written by the indexing model from the issue text.