Add additional license information and format markdown for GitHub

[TimidRobot]

Add additional license information and format markdown for GitHub

(formatting is pretty subjective, let me know if you want anything modified, removed, etc.)

[davidwolfpaw]

All of these look good, I'll merge it in!

The only note on the readme is that I'd left it at my wordpress.org profile handle, since their repository uses that to link it to the WordPress user account.

But this is also only taken from the readme.txt, and I created readme.md specifically for Github, so all good!