Mention MIT license in README.
Mention MIT license in README.

Closes #556

mcanthony/paper.js,nancymark/paper.js,mcanthony/paper.js,ClaireRutkoske/paper.js,nancymark/paper.js,nancymark/paper.js,EskenderDev/paper.js,iconexperience/paper.js,lehni/paper.js,byte-foundry/paper.js,byte-foundry/paper.js,EskenderDev/paper.js,lehni/paper.js,lehni/paper.js,byte-foundry/paper.js,iconexperience/paper.js,iconexperience/paper.js,ClaireRutkoske/paper.js,EskenderDev/paper.js,ClaireRutkoske/paper.js,mcanthony/paper.js