Add MIT license
Add MIT license

This matches the license used by the govuk_frontend_toolkit_npm
package.json file.

https://github.com/alphagov/govuk_frontend_toolkit_npm/blob/master/packa
ge.json#L13

alphagov/govuk_elements,alphagov/govuk_elements,gavboulton/govuk_elements,gavboulton/govuk_elements,alphagov/govuk_elements,joelanman/govuk_elements,alphagov/govuk_elements,gavboulton/govuk_elements,joelanman/govuk_elements,gavboulton/govuk_elements