Add LICENSE file to uikit-site

[mpezzi]

First off, thank you all for such an awesome project.

Secondly, I'm curious of what type of license is used for uikit-site. Is it the same MIT license as uikit?

The reason I'm asking is because we've chosen uikit as a framework and would like to provide our team members with a documentation site, but with the various style tweaks as requested by our design team. Specifically, we'd like to reuse your existing code example pages for components.

If we could add a LICENSE file so everyone is clear on what we can or cannot use that would be helpful.

Thanks!