nixos/release-small.nix: add instructions for running individual jobs

# This jobset is used to generate a NixOS channel that contains a

# small subset of Nixpkgs, mostly useful for servers that need fast

# security updates.

#

# Individual jobs can be tested by running:

#

#   nix-build nixos/release-small.nix -A <jobname>

#

{ nixpkgs ? { outPath = (import ../lib).cleanSource ./..; revCount = 56789; shortRev = "gfedcba"; }

, stableBranch ? false

, supportedSystems ? [ "aarch64-linux" "x86_64-linux" ] # no i686-linux