git-standup: add sigmanificient to maintainers

pkgs/applications/version-management/git-standup/default.nix

···
     description = "Recall what you did on the last working day";
     homepage = "https://github.com/kamranahmedse/git-standup";
     license = licenses.mit;
-    maintainers = [ ];
+    maintainers = with lib.maintainers; [ sigmanificient ];
     platforms = platforms.all;
     mainProgram = "git-standup";
   };