add some docs for rtorrent; add qbittorrent as another option

nix/home/programs/qbittorrent/default.nix

@@ -0,0 +1,21 @@
+{ pkgs }:
+{
+  environment.systemPackages = with pkgs; [ qbittorrent-nox ];
+
+  systemd = {
+    packages = [ pkgs.qbittorrent-nox ];
+
+    services."qbittorrent-nox@fc" = {
+      enable = true;
+      serviceConfig = {
+        Type = "simple";
+        User = "rtorrent";
+        ExecStart = "${pkgs.qbittorrent-nox}/bin/qbittorrent-nox";
+      };
+      wantedBy = [ "multi-user.target" ];
+    };
+  };
+
+  networking.firewall.allowedTCPPorts = [ 8080 ];
+
+}