hswaw/signage: add development tip

Change-Id: I6dff93498ba2de959f1c7760a21c0972c2277037
Reviewed-on: https://gerrit.hackerspace.pl/c/hscloud/+/1336
Reviewed-by: q3k <q3k@hackerspace.pl>
diff --git a/hswaw/signage/default.nix b/hswaw/signage/default.nix
index 0ac8c66..5909659 100644
--- a/hswaw/signage/default.nix
+++ b/hswaw/signage/default.nix
@@ -34,6 +34,8 @@
         EOF
         chmod +x $out/bin/signage
       '';
+
+      passthru.runtime = love12;
     };
   in signage;