Add option for custom repositories about apktrack

[JusticeRage]

I've toyed with the idea of letting the user type in his own repository, but I don't think this is going to work. Typing regular expressions on a smartphone keyboard is a total pain.
To add more sources to ApkTrack, there are two ways to go as of now.

1. Editing this file and submitting a pull request.
2. Telling me about a repository and I will edit the aforementioned file myself.

I will look into GuardianProject, but for now, I keep being redirected to this page and can't access any app information.

[Mosrite]

I'm really not an expert. I added "https://guardianproject.info/repo/" as additional repository on F-Droid and it works fine. I have no idea how F-Droid handles the repository.
Another interesting repository might be the one of I2P: https://f-droid.i2p.io/repo

[JusticeRage]

The latest beta APK includes Guardian Project and I2P as available update sources!
Let me know if this fixes the issue for you!