Add open-source license

[pumpkinhasapatch]

It is recommended to add a LICENSE file to all repositories, to allow other people to use and redistribute your code and get proper credit for your work. Seeing this is a small project that has no license for several years, I guess the owners don't care about copyrights, but it's still good practice to use a license anyway.

See https://choosealicense.com for a list of popular open-source licenses and more information.

Several forks of this paint editor already exist, so adding a license now might cause legal issues when merging code. I'm not really sure how to handle this...