Separate license and contribution policy

The framework currently includes this language:

> The license should also include a “terms of contribution” that clearly states that any code contributions are covered under the overarching license.

It may not be a good practice to edit existing licenses, as it's not expected. Instead, a common alternative approach is to include a `CONTRIBUTING.md` file, stating that contributors agree to license their work in the same manner, in the root of the repository in addition to the `LICENSE.md` file. [Here's](https://github.com/18F/18f.gsa.gov/) an example. As a bonus, GitHub will add a link to that file at the top of the forms for issues and pull requests, so potential contributors will be informed that the repository has a contribution policy. Under this approach, it's a good idea to reference both documents in `README.md`.