Add LICENSE file

See #310. Content is licensed under OGL. Source code is licensed under MIT.
Distilling [Pivotal #68531256](https://www.pivotaltracker.com/story/show/68531256): any work created in the course of 'his' duties as a Civil Servant is governed by [§163 of the Copyright, Designs and Patents Act (1988, as amended)](http://www.legislation.gov.uk/ukpga/1988/48/section/163). But, IANAL (anywhere, at all), so we should get this clarified by someone who acts as such authority within GDS or Cabinet Office.
 
Related: @alexmuller [points out](https://twitter.com/alexmuller/status/521779309422600192) an example of other entities (but all GDS/Government-based) claiming copyright in other repositories.

I'll try and make some enquiries.