Added documentation

SDT/schema4.0/docs/SDT_Components.md

@@ -69,6 +69,7 @@ It can also be used to collect all specified [ModuleClasses](#ModuleClasses), [D
 - **Imports** : XML import/include of other XML files / SDT templates. Optional.  
 Note, that when using **Imports** one must include the following namespace in the Domain-Element definition, as shown in the example below: ```xmlns:xi="http://www.w3.org/2001/XInclude"```
 - **[DataTypes](#DataType)** : A list of *DataTypes* that are global to the whole domain. They can be referenced from other definitions by the [Extend](#Extend) element. Optional.
+- **[SubDevices](#SubDevice)** : a List of *SubDevice* components. They can be referenced from other SubDevice definitions by the [Extend](#Extend) element. Optional.
 - **[ModuleClasses](#ModuleClass)** : A list of those *ModuleClass* components that are global to the whole domain. Optional.
 - **[DeviceClasses](#DeviceClass)** : a List of *DeviceClass* components. Optional.
 - **[ProductClasses](#ProductClass)** : a List of *ProductClass* components. Optional.