Add LICENSE

Followed the pattern of nodejs/libuv. License is MIT "expat".

Moving forward with MIT, but I think it is advisable to to ask contributors to sign a [CLA](http://en.wikipedia.org/wiki/Contributor_License_Agreement) to move contributions to MIT. 

Useful discussion of jQuery's move to MIT, which required a CLA: https://news.ycombinator.com/item?id=4515362

Neovim CLA: https://docs.google.com/forms/d/1u54bpbwzneDIRltFx1TGi2evKxY3w0cOV3vlpj8DPbg/viewform?usp=send_form

CLA means: 

- You retain the copyright to your work
- You are not obligated to support your contribution
- Neovim is allowed to use your work
- Neovim is allowed to relicense your work

Any contributions that are not approved for MIT, remain under the Vim license. Exiting Vim code and Vim patches also remain under the Vim license.