Add message about CC0 License to Editor (revision)

Builds on #1794 to take @tilgovi's comments on board. As I agree, if we're going to include this information on the editor then it should be very clearly explained in as simple terminology as possible.

I've used the suggested text and included the CC-0 icons and retained the link to the full documentation.

![screen shot 2014-12-12 at 12 52 34](https://cloud.githubusercontent.com/assets/47144/5411211/2c0d38c8-81fe-11e4-9304-48c4b4ed16b9.png)