add boilerplate 18F files

[afeld]

https://github.com/18F/open-source-policy/blob/master/practice.md#how-to-license-18f-repos

Not sure how that should interact with the MIT license...?

/cc @rogeruiz

[rogeruiz]

I'm comfortable with adding 18F files. Not sure how we should include the reference to have the original license included. ¯_(ツ)_/¯

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

[brittag]

I'll make a PR for this! Here's how we worked it out for a different forked project - https://github.com/18F/retext-18f/blob/master/LICENSE.md

[afeld]

Fixed by #19.