The primary reason: make GitHub detect that we use Apache License 2.0
They do it via https://github.com/licensee/licensee Ruby library (gem).
Our COPYRIGHT file contains a part of the Apache License, which should
be added to a source file, not the license or copyright information itself,
which confuses the library.
Instead, the recommended way is to create a NOTICE file which references
license of the code and its bundled dependencies.
