Licensing: Update examples & add License file

From HN: https://news.ycombinator.com/item?id=15910300

Hey, great work! Just a heads up about licensing. JavaScript code fragments that you use inside the compiler to produce the final output are technically part of the source code, and thus the output is also GPLv3 licensed as a derived work.

That's a known GPL quirk, and other GPL-licensed compilers like GCC use a separate variant of GPL to avoid that collision.

You can see how GNU Bison does it: https://git.savannah.gnu.org/cgit/bison.git/tree/src/parse-g... since they put fragments of Bison code to the final generate source file. That's exactly what Joy does to generate JavaScript, so you should put similar notice to the repository.

Also, LICENCE file is not there.