rename the license-file from COPYING to LICENSE

Back when the license file was added COPYING seemed like a more
common file-name for license files. However, times seems to have
changed. GitHub recommends[1] either LICENSE.txt or LICENSE instead.

So let's choose LICENSE.txt instead. Using the variant with the .txt
suffix is a tiny bit friendlier to Windows users, where file extensions
makes a difference.

[1]: https://help.github.com/articles/open-source-licensing/#where-does-the-license-live-on-my-repository