Nixpkgs merge bot testing and plan

After receiving a lot of concerned feedback in a previous attempt, it’s time to clear things up and give it another chance. Starting today, the Nixpkgs merge bot will be enabled in dry-run mode. This means that it is not privileged to actually merge anything at all just yet, and instead only prints a comment explaining what it would’ve done if merges were enabled. The people behind this effort are @Lassulus and @Mic92. This announcement was written with @infinisil’s help, who’s also in favor...