Add better license notification

[ian-kelling]

Just having the license file does not make it completely clear that all files are under that license. State that fact in your README.md, and state whether it is gplv3 or later (most people prefer this) or just gplv3. And if you really want to be fully correct the end of the gplv3 text it says you should put text at the top of each file to say it is gplv3.