Add missing License

[Stevenin nicolas]

Hello,

Could you add the License file please ?

Thank you

[Mark Rushakoff]

The README indicates an MIT license, but the github UI isn't picking it up. ping @bmizerany.

[Andy Maloney]

(probably related)

Without the license, the docs the README point at say:

Documentation not displayed due to license restrictions.

[Ruben Klink]

You need to add a dedicated LICENSE file otherwise it is not explicit enough. PR #43 is already doing that.

[Blake Mizerany]

Done.