Add MIT licence, owned by HMG/GDS

This mirrors the licences used in [many](https://github.com/alphagov/govuk_frontend_toolkit/blob/master/LICENCE) [of](https://github.com/alphagov/frontend/blob/master/LICENCE.txt) [our](https://github.com/alphagov/static/blob/master/LICENCE) [other](https://github.com/alphagov/smart-answers/blob/master/LICENSE.md) [repositories](https://github.com/alphagov/router/blob/master/LICENSE).

Fixes #1683