Add a LICENSE file to the project

Problem

Code hosted on Drupal is explicitly licensed under GPLv2 or later: https://www.drupal.org/about/licensing

However, this is not apparent when you find our code through GitHub.

Solution

Add the correct LICENSE file to the repository.

Drupal's license file can be found here: https://github.com/drupal/drupal/blob/8.6.x/core/LICENSE.txt