Add licensing information to the FAQ page.

I would like to upload screenshots of tests run using your application on Wikipedia (Wikimedia Commons specifically). After coming to GitHub here and browsing the files for: **The source code for jsPerf is copyright © Mathias Bynens and dual-licensed under the `MIT` and `GPL` licenses.**, I now know what license it is under, but it would have been much more convenient for it to be on the FAQ page or at the bottom of my test page.

Start by locating the FAQ page and reviewing its existing licensing-related content. Add the stated dual MIT and GPL licensing information there, and consider whether the requested test-page location is part of the same change; done means visitors can find the license without browsing source files.

Written by the indexing model from the issue text.