Include the text of the GPLv3 at the bottom of the `LICENSE.md` file

[DilumAluthge]

If you're interested in registering your Julia packages in the Julia General registry, you might notice that the AutoMerge CI check has trouble parsing the license file.

The easiest way to fix AutoMerge is to include the text of the GPLv3 license at the bottom of the LICENSE.md file. Then AutoMerge will detect the GPL license text, and the check will pass.