Check signature using keyring

Curently listings for signed AppImages say something like

> "Signature made Sat 20 Jan 2018 07:56:54 AM UTC using DSA key ID 462E2AF2 Can't check signature: No public key"

We need to implement some trusted keychain. Question: how can we crowd-source trust?

Reference:
https://github.com/AppImage/appimage.github.io/pull/389#issuecomment-368621813