Added Options document

docs/en/Options.md

@@ -1,4 +1,73 @@
 # Options
 
-TODO!
+Microsoft has introduced [the options pattern](https://docs.microsoft.com/en-us/aspnet/core/fundamentals/configuration/options) that is used to configure a group of settings used by the framework services. This pattern is implemented by the [Microsoft.Extensions.Options](https://www.nuget.org/packages/Microsoft.Extensions.Options) NuGet package, so it is usable by any type of applications in addition to ASP.NET Core based applications.
 
+ABP framework follows this option pattern and defines options classes to configure the framework and the modules (they are explained in the documents of the related feature).
+
+Since [the Microsoft documentation](https://docs.microsoft.com/en-us/aspnet/core/fundamentals/configuration/options) explains the pattern in detail, no reason to repeat all. However, ABP adds a few more features and they will be explained here.
+
+## Configure Options
+
+You typically configure options in the `ConfigureServices` of the `Startup` class. However, since ABP framework provides a modular infrastructure, you configure options in the `ConfigureServices` of your [module](Module-Development-Basics.md). Example:
+
+````csharp
+public override void ConfigureServices(ServiceConfigurationContext context)
+{
+    context.Services.Configure<AbpAuditingOptions>(options =>
+    {
+        options.IsEnabled = false;
+    });
+}
+````
+
+* `AbpAuditingOptions` is a simple class defines some properties like `IsEnabled` used here.
+* `AbpModule` base class defines `Configure` method to make the code simpler. So, instead of `context.Services.Configure<...>`, you can directly use the `Configure<...>` shortcut method.
+
+If you are developing a reusable module, you may need to define an options class to allow developers to configure your module. In this case, define a plain options class as shown below:
+
+````csharp
+public class MyOptions
+{
+    public int Value1 { get; set; }
+    public bool Value2 { get; set; }
+}
+````
+
+Then developers can configure your options just like the `AbpAuditingOptions` example above:
+
+````csharp
+public override void ConfigureServices(ServiceConfigurationContext context)
+{
+    Configure<MyOptions>(options =>
+    {
+        options.Value1 = 42;
+        options.Value2 = true;
+    });
+}
+````
+
+* In this example, used the shortcut `Configure<...>` method.
+
+### Get the Option Value
+
+Whenever you need to get the value of an option, [inject](Dependency-Injection.md) the `IOptions<TOption>` service into your class and use its `.Value` property. Example:
+
+````csharp
+public class MyService : ITransientDependency
+{
+    private readonly MyOptions _options;
+    
+    public MyService(IOptions<MyOptions> options)
+    {
+        _options = options.Value; //Notice the options.Value usage!
+    }
+
+    public void DoIt()
+    {
+        var v1 = _options.Value1;
+        var v2 = _options.Value2;
+    }
+}
+````
+
+Read [the Microsoft documentation](https://docs.microsoft.com/en-us/aspnet/core/fundamentals/configuration/options) for all details of the options pattern.