Create LICENSE file

[vinceAmstoutz]

I took the license file from the other API Platform repos.

[dunglas]

Unlike the code, the docs are under CC by 3.0, not under MIT.
By the way, we could upgrade to CC 4.0. If we do so, we must update the license on the website too.

[vinceAmstoutz]

Unlike the code, the docs are under CC by 3.0, not under MIT. By the way, we could upgrade to CC 4.0. If we do so, we must update the license on the website too.

Here's what I've done with the CC-BY-4.0 template. If it's okay for you Kévin, I'll also make a PR for the website.

[dunglas]

On for me! Thanks!!