[BridgeAbstract] Add 'getDescription' function

BridgeAbstract.md

@@ -3,6 +3,7 @@
 To create a new Bridge extending `BridgeAbstract` you must specify some [basic metadata](#basic-metadata) implement following functions:
 
 - [`collectData`](#the-collectdata-function) (**required**)
+- [`getDescription`](#the-getdescription-function)
 - [`getName`](#the-getname-function)
 - [`getURI`](#the-geturi-function)
 
@@ -169,6 +170,18 @@ Parameter | ATOM | HTML | (M)RSS
 `content`|X|X|X
 `enclosures`|X|X|X
 
+## The `getDescription` function
+
+This function returns the description of a bridge.
+
+**Notice:** **RSS-Bridge** will by default return `const DESCRIPTION` which is defined in the bridge, so you only have to implement this function if you require different behavior!
+
+```PHP
+	public function getDescription(){
+		return self::DESCRIPTION;
+	}
+```
+
 ## The `getName` function
 
 This function returns the name of the bridge as it will be displayed on the main page of **RSS-Bridge** or on top of the feed output (HTML, ATOM, etc...).