Create LICENSE file

[kuba-kubula]

Due to the text of the MIT License, the whole License text must be packaged and distributed alongside the code.

But the slick library is awesome and used by some cool projects (juice, inline-css) and they depend on this package, but since the License file is missing, it can't be used by enterprise / my employer due to possible legal issues.

Can you please merge & the publish a new version? Thanks a ton!

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Copyright holder taken from https://github.com/subtleGradient/slick/blob/master/package.yml