Stared to document the feature system

docs/en/Features.md

@@ -1,3 +1,47 @@
 # Features
 
-TODO
+ABP Feature system is used to **enable**, **disable** or **change the behavior** of the application features **on runtime**.
+
+Feature system was originally designed to control the tenant features in a **multi-tenant** application. However, it is **extensible** and capable of determining features by any condition.
+
+> The feature system is implemented with the [Volo.Abp.Features](https://www.nuget.org/packages/Volo.Abp.Features) NuGet package. Most of the times you don't need to manually [install it](https://abp.io/package-detail/Volo.Abp.Features) since it comes pre-installed with the [application startup template](Startup-Templates/Application.md).
+
+## Checking for the Features
+
+Before starting to explain how to define features, let's see how to check a feature in your application code.
+
+### RequiresFeature Attribute
+
+`[RequiresFeature]` attribute (defined in the `Volo.Abp.Features` namespace) is used to declaratively check if a feature is enabled or not.
+
+**Example: Check if the current user/tenant has "PDF Reporting" feature enabled**
+
+```csharp
+public class ReportingAppService : ApplicationService, IReportingAppService
+{
+    public async Task<CsvReportResultDto> GetCsvReportAsync()
+    {
+        throw new System.NotImplementedException();
+    }
+
+    [RequiresFeature("MyApp.PdfReporting")]
+    public async Task<PdfReportResultDto> GetPdfReportAsync()
+    {
+        throw new System.NotImplementedException();
+    }
+}
+```
+
+* `RequiresFeature(...)` simply gets a feature name to check if it is enabled or not. If not enabled, an authorization [exception](Exception-Handling.md) is thrown and a proper response is returned to the client side.
+* `[RequiresFeature]` can be used for a **method** or a **class**. When you use it for a class, all the 
+* `RequiresFeature` may get multiple feature names, like `[RequiresFeature("Feature1", "Feature2")]`. In this case ABP checks if current user/tenant has any of the features enabled. Use `[RequiresFeature("Feature1", "Feature2", RequiresAll = true)]` to force to allow only if all of the features are enabled.
+* Multiple usage of `[RequiresFeature]` attribute is enabled, so it checks all of them.
+
+#### About the Interception
+
+ABP Framework uses the interception system to make the `[RequiresFeature]` attribute working. So, it can work with any class that is injected from the [dependency injection](Dependency-Injection.md).
+
+However, there are **some rules should be followed** in order to make it working;
+
+* If you are **not injecting** the service over an interface (like `IMyService`), then the methods of the service must be `virtual` (otherwise, [dynamic proxy / interception](Dynamic-Proxying-Interceptors.md) system can not work).
+* Only `async` methods (methods returning a `Task` or `Task<T>`) are intercepted.