Document how to start a server in Exporting for dedicated servers

This mainly applies to projects where both the client and server
are implemented as part of the same project.

getting_started/workflow/export/exporting_for_dedicated_servers.rst

@@ -82,6 +82,34 @@ different name, you can specify the path to the PCK file using the
 
     ./godot-server --main-pack my_project.pck
 
+Starting the dedicated server
+-----------------------------
+
+If both your client and server are part of the same Godot project, you will have
+to add a way to start the server directly using a command-line argument. This
+can be done by adding the following code snippet in your main scene (or a
+singleton)'s ``_ready()`` method::
+
+    if "--server" in OS.get_cmdline_args():
+        # Run your server startup code here...
+        # Using this check, you can start a dedicated server by running
+        # a Godot binary (headless or not) with the `--server` command-line argument.
+        pass
+
+Alternatively, you can make the dedicated server always start up if a headless
+or server binary is detected::
+
+    # Note: Feature tags are case-sensitive! It's "Server", not "server".
+    if OS.has_feature("Server"):
+        # Run your server startup code here...
+        # Note that using this check may break unit testing scripts when
+        # running them with headless or server binaries.
+        pass
+
+If your client and server are separate Godot projects, your server should most
+likely be configured in a way where running the main scene starts a server
+automatically.
+
 Next steps
 ----------