We've just released GitHub for Mac 1.2.5 and it comes with a new update panel that helps you see what we're shipping with each release. It also contains a handy Relaunch button which will close GitHub for Mac and relaunch the update right back to where you were!
You'll see the popover the first time you update after downloading 1.2.5. It won't appear in the version you have now.
The new update should download automatically, but if you'd like to manually check for an update you can go to GitHub → Check For Updates. If you haven't installed GitHub for Mac yet, you can download it over at mac.github.com