You can add files like this to your repository with Git! You’re right that you can’t edit them with Anvil, but if you check the repository out locally, create a LICENSE file, then git add it to the repository, commit it, and push it back to Anvil, it will stay there – and whenever you check that repository out again, or push it to Github, there it will be 
(If you’re putting it on Github, I would recommend doing the same with a README.md)