LICENSE file could be added to ArduinoCore-avr

A LICENSE file in the ArduinoCore-avr repository would make it easier to be sure about the license status of the included files. Currently users have to check the license information in each individual file before e.g. copying / forking this repository. Otherwise there is no way to be sure that the information "the C/C++ microcontroller libraries are under the LGPL" ([see Arduino FAQ](https://www.arduino.cc/en/Main/FAQ)) refers to the whole content of this repository.
You could start by checking whith your lawyer if LGPL 2.1 or newer would be the best choice for the LICENSE file or if the LICENSE file has to specify the license for each file individually.

Start by reviewing the proposed LICENSE file, the individual file license notices, and the Arduino FAQ reference in the issue. Confirm with project maintainers or legal counsel whether a repository-wide LGPL 2.1-or-newer license is appropriate; done means the licensing scope and required LICENSE contents are decided and documented.

Written by the indexing model from the issue text.