nixos/nebula: Add release notes

[Jaculabilis]

Motivation for this change

The Nebula module is new in 21.05 via #111518.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[lukegb]

@NixOS/release-engineers

[Ma27]

Aren't those additions generated via a script before the release?

[Jaculabilis]

@Ma27 There are already some services listed here, so I figured at least some things are added manually. Is there an existing process documented or hinted at somewhere?

[jonringer]

Aren't those additions generated via a script before the release?

If you mean, I run some scripts and then manually prune and improve them. Yes. They are generated :)