Add copyright notice and license of document.

Upon agreement of all the authors mentioned in the earlier commit and
listed herein, the license of the document is as this commit describes.

First publication of GPL Enforcement Principles.

Bradley M. Kuhn wrote the first draft of these principles on 23 May
2015. They were from there rewritten collaboratively over the following
months with the participation (in chronological order) of Karen
M. Sandler, W. John Sullivan, and Allison Randal.

This commit is the first publication of the principles on 1 October
2015.