Update Proton v0.8.0 releases

Hello there,

in this release I declare the flatpak integration as stable enough.

Have fun!

What's Changed

• Update README.md by @heuwerk in #19
• Update README.md by @heuwerk in #20
• First implementation of flatpak support by @heuwerk in #22
• Update README.md by @heuwerk in #23

Full Changelog: v0.7.0...v0.8.0