Please add a LICENSE file

[jq170727]

I think your treemap does a good job summarizing contributions I've made to some repositories and I think it could aid me in my job search if I could incorporate some code derived from treemap.js into an interactive portfolio I'm building. But while refactoring some things to make the code play nicer with React I noticed there's no LICENSE file. It would be good to have one so folks like me can know if they can to use code derived from your treemap.js or will otherwise need to find an alternative.