Somewhat off topic, but related since it's not documented anywhere: you can put ads on your GitHub Pages install.

I'm not positive to the extent and what internal rules may apply. But out of curiosity I emailed the GH legal dept and they said it was fine (after checking around and getting back a couple weeks later).

So if you want to monetize your open source projects with Adsense or otherwise, that's actually an option! (Not that I have or ever will)