Add "License.MD"

[MGradenko]

I would love to use Klass, however, without a license, the Moodle theme/plugin is "All Rights Reserved" and thus not available for use (or modification and redistribution) without expressed permission by the author (copyright holder). Without a license that allows continued use, those using Klass would be subject to any changes in use or permissions required by the copyright holder. Licensing also helps the Klass user-community to contribute back to the project (plugin/theme).

Github recommends using an open source software license for publically available repositories where the copyright holder allows use, modification, and redistribution, see, "Licensing a repository."

Thank you for your consideration.