Add an explicit license.

[Skenvy]

At the moment, the closest to this is the README saying

You're welcome to use this repo to test Dependabot functionality.

But it would be nice if the licensing was less vague and ephemeral.

I ask because I'd like to duplicate the files here in a set of tests to run github's linguist on. Linguist can't run on submodules, and I'd like for the unit tests to not have to rely on an external repo, or multiple versions of it given the pinned versions that the smoke tests here guarentee to be runnable with dependabot.