* Support comp.nix.
* Re-order overlays and modules dependency-last for sanity.
* Re-export `xmonad.modernise`.
NIX.md:
* Document comp.nix.
* Simplify example system flake, removing direct dependency on the
xmonad flake.
* Advise sane module ordering.
* Include and document `modernise` workaround.
* Clarify user-substituted `<foo>` values.
.gitignore:
* Include nix 'result' symlink.
With "nix build", just the build of xmonad-contrib itself takes 3
minutes (it builds twice, the second build with profiling enabled), so
it ends up being almost 6 minutes in total, making this workflow the
slowest one.
