add GPG signatures

[dashfan]

Good example: https://github.com/Bertrand256/electrum-dash/releases

Quote from qwizzie: "...Same for Platform releases, they should also have shasum and .asc files. Core releases have this fully covered on Github, but i think this should also extend to https://www.dash.org/downloads/ (not just .asc file for desktop installer but sha256 links as well). In the end sha + .asc should cover Platform releases, Core releases (incl dash.org/downloads) and DET. If Dash is serious about providing release security and verification to its end users..."