docs(sterni/blërg): list dependencies in README

Change-Id: Iceacf406bef71529cb29e8adcf9fe1f713187e42
Reviewed-on: https://cl.tvl.fyi/c/depot/+/13103
Autosubmit: sterni <sternenseemann@systemli.org>
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

users/sterni/blërg/default.nix

@@ -1,12 +1,14 @@
 { depot, pkgs, lib, ... }:
 
 let
+  # update README.md when changing this
   runtimeDependencies = [
     depot.users.sterni.mn2html
     pkgs.mblaze
     pkgs.execline # execline-cd
   ];
 
+  # … and this
   buildInputs = [
     pkgs.cbqn
   ];