Add link to repo on 'about' screen

[keunes]

Hi there,

I love the simple and modern interface of the app :) I was worried that it was an experiment that had died (since the last update on Google Play was almost a year ago), but I'm happy to see that this is still being worked on here on GitHub. So just a word of encouragement to keep going ;)

Now, what I actually wanted to request: in the about screen it'd be good to include a link to this repo, so it's easier for users to find the project to contribute, report bugs and make feature requests.