Added section: Creating a Simple Plug-In

docs/en/PlugIn-Modules.md

@@ -53,4 +53,71 @@ There are two more built-in Plug-In Source implementations:
 * `PlugInSources.AddFiles()` gets a list of assembly (typically `dll`) files. This is a shortcut of using `FilePlugInSource` class.
 * `PlugInSources.AddTypes()` gets a list of module class types. If you use this, you need to load the assemblies of the modules yourself, but it provides flexibility when needed. This is a shortcut of using `TypePlugInSource` class.
 
-If you need, you can create your own `IPlugInSource` implementation and add to the `options.PlugInSources` just like the others.
+If you need, you can create your own `IPlugInSource` implementation and add to the `options.PlugInSources` just like the others.
+
+## Creating a Simple Plug-In
+
+Create a simple **Class Library Project** in a solution:
+
+![simple-plugin-library](images/simple-plugin-library.png)
+
+You can add ABP Framework packages you need to use in the module. At least, you should add the `Volo.Abp.Core` package to the project:
+
+````
+Install-Package Volo.Abp.Core
+````
+
+Every [module](Module-Development-Basics.md) must declare a class derived from the `AbpModule`. Here, a simple module class that resolves a service and initializes it on the application startup:
+
+````csharp
+using Microsoft.Extensions.DependencyInjection;
+using Volo.Abp;
+using Volo.Abp.Modularity;
+
+namespace MyPlugIn
+{
+    public class MyPlungInModule : AbpModule
+    {
+        public override void OnApplicationInitialization(ApplicationInitializationContext context)
+        {
+            var myService = context.ServiceProvider
+                .GetRequiredService<MyService>();
+            
+            myService.Initialize();
+        }
+    }
+}
+````
+
+`MyService` can be any class registered to [Dependency Injection](Dependency-Injection.md) system, as show below:
+
+````csharp
+using Microsoft.Extensions.Logging;
+using Volo.Abp.DependencyInjection;
+
+namespace MyPlugIn
+{
+    public class MyService : ITransientDependency
+    {
+        private readonly ILogger<MyService> _logger;
+
+        public MyService(ILogger<MyService> logger)
+        {
+            _logger = logger;
+        }
+
+        public void Initialize()
+        {
+            _logger.LogInformation("MyService has been initialized");
+        }
+    }
+}
+````
+
+Build the project, open the build folder, find the `MyPlugIn.dll`:
+
+![simple-plug-in-dll-file](images/simple-plug-in-dll-file.png)
+
+Copy `MyPlugIn.dll` into the plug-in folder (`D:\Temp\MyPlugIns` for this example).
+
+If you have configured the main application like described above (see Basic Usage section), you should see the `MyService has been initialized` log in the application startup.