Highlight update process in README

[tooomm]

• Use admonition to highlight section
• Add note on new, automated update process - thanks to Update runtime, update protobuf, add x-checker-data #61

[flathubbot]

Started test build 173599

[flathubbot]

Build 173599 successful
To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/156728/io.github.Cockatrice.cockatrice.flatpakref