Documented basic usage of the plug-in modules.

docs/en/PlugIn-Modules.md

@@ -1,3 +1,56 @@
 # Plug-In Modules
 
-TODO
+It is possible to load [modules](Module-Development-Basics.md) as plug-ins. That means you may not reference to a module's assembly in your solution, but you can load that module in the application startup just like any other module.
+
+## Basic Usage
+
+`IServiceCollection.AddApplication<T>()` extension method can get options to configure the plug-in sources.
+
+**Example: Load plugins from a folder**
+
+````csharp
+using Microsoft.AspNetCore.Builder;
+using Microsoft.Extensions.DependencyInjection;
+using Volo.Abp.Modularity.PlugIns;
+
+namespace MyPlugInDemo.Web
+{
+    public class Startup
+    {
+        public void ConfigureServices(IServiceCollection services)
+        {
+            services.AddApplication<MyPlugInDemoWebModule>(options =>
+            {
+                options.PlugInSources.AddFolder(@"D:\Temp\MyPlugIns");
+            });
+        }
+
+        public void Configure(IApplicationBuilder app)
+        {
+            app.InitializeApplication();
+        }
+    }
+}
+````
+
+* This is the `Startup` class of a typical ASP.NET Core application.
+* `PlugInSources.AddFolder` gets a folder path and to load assemblies (typically `dll`s) in that folder.
+
+That's all. ABP will discover the modules in the given folder, configure and initialize them just like regular modules.
+
+### Plug-In Sources
+
+`options.PlugInSources` is actually a list of `IPlugInSource` implementations and `AddFolder` is just a shortcut for the following expression:
+
+````csharp
+options.PlugInSources.Add(new FolderPlugInSource(@"D:\Temp\MyPlugIns"));
+````
+
+> `AddFolder()` only looks for the assembly file in the given folder, but not looks for the sub-folders. You can pass `SearchOption.AllDirectories` as a second parameter to explore plug-ins also from the sub-folders, recursively.
+
+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 implement your own `IPlugInSource` implementation and add to the `options.PlugInSources` just like the others.