docs: provide and document a quick way to apply a change to nixpkgs (#90)

Only one way is documented, when a typical overlay for a single package is used, which brings its own dependencies from a given nixpkgs commit.

Co-authored-by: Alexander Tomokhov <alexoundos@gmail.com>
Reviewed-on: https://git.selfprivacy.org/SelfPrivacy/selfprivacy-nixos-config/pulls/90
Reviewed-by: Inex Code <inex.code@selfprivacy.org>
Co-authored-by: Alexander Tomokhov <alexoundos@selfprivacy.org>
Co-committed-by: Alexander Tomokhov <alexoundos@selfprivacy.org>

configuration.nix

@@ -149,6 +149,9 @@ in
     # allowed-uris = [];
     allow-dirty = false;
   };
+  nixpkgs.overlays = [
+    (import ./overlay.nix config.nixpkgs.hostPlatform.system)
+  ];
   services.journald.extraConfig = "SystemMaxUse=500M";
   boot.kernel.sysctl = {
     "net.ipv4.ip_forward" = 1; # TODO why is it here by default, for VPN only?