[RDY] Add LICENSE

Followed the pattern of nodejs/libuv. License is ~~MIT "expat"~~ Apache 2.0.

Moving forward with ~~MIT~~ Apache 2.0, but I think it is advisable to to ask contributors to sign a [CLA](http://en.wikipedia.org/wiki/Contributor_License_Agreement) to move existing contributions to ~~MIT~~ Apache 2.0. Any contributions that are not approved for ~~MIT~~ Apache 2.0, remain under the Vim license. Existing Vim code and Vim patches also remain under the Vim license.

Reference:
- [discussion of jQuery's move to MIT, which required a CLA](https://news.ycombinator.com/item?id=4515362)
- [merits and limits of CLA](http://julien.ponge.org/blog/in-defense-of-contributor-license-agreements/)

[Neovim CLA](https://docs.google.com/forms/d/1u54bpbwzneDIRltFx1TGi2evKxY3w0cOV3vlpj8DPbg) summary: 

- You retain the copyright to your work.
- You are not obligated to support your contribution.
- Neovim is allowed to use your work under the Apache 2.0 license.
- Neovim may relicense your work under the following conditions (this does not affect your copyright):
    - Any license change will be announced 1 month in advance, on the neovim mailing, website, and the email address you provide. If no objections are raised then that is interpreted as approval of the license change.