Extend nixpkgs dunst systemd service

The service can not be enabled, as it doesn't contain the ``[Install]``.
The overlay patches that two lines to the service.

That's to my current knowledge.

overlays/dunst/default.nix

@@ -0,0 +1,7 @@
+self: super: {
+  dunst = super.dunst.overrideAttrs(oldAttrs: rec {
+    patches = [
+      ./systemd-service.patch
+    ];
+  });
+}

overlays/dunst/systemd-service.patch

@@ -0,0 +1,11 @@
+diff --git a/dunst.systemd.service.in b/dunst.systemd.service.in
+index 9fa168c..5ac2217 100644
+--- a/dunst.systemd.service.in
++++ b/dunst.systemd.service.in
+@@ -7,3 +7,6 @@ PartOf=graphical-session.target
+ Type=dbus
+ BusName=org.freedesktop.Notifications
+ ExecStart=##PREFIX##/bin/dunst
++
++[Install]
++WantedBy=default.target

readme.rst

@@ -23,6 +23,16 @@ The idea: Provide a configuration instead of some state.
 It can be installed via ``nix-env -i my-packages``
 which also "updates" state accordingly to the current defined set.
 
+systemd units
+-------------
+
+Some derivations will provide ready to use systemd units.
+Those are most likely placed within ``~/.nix-profile/lib/systemd/*``.
+
+One can symlink them into ``~/.config/systemd``.
+
+E.g. ``~/.config/systemd/user/dunst.service -> ~/.nix-profile/lib/systemd/user/dunst.service``
+
 Update
 ------