Update usage.md

Author: Javierlj

doc/usage.md

@@ -44,6 +44,54 @@ The received data is a DataStream of objects of the class **`NgsiEvent`**. This
 
         -   **`metadata`**: Additional metadata.
 
+
+### NGSILDSource
+
+-   Import dependency.
+
+```scala
+import org.fiware.cosmos.orion.flink.connector.{NGSILDSource}
+```
+
+-   Add source to Flink Environment. Indicate what port you want to listen to (e.g. 9001).
+
+```scala
+val env = StreamExecutionEnvironment.getExecutionEnvironment
+val eventStream = env.addSource(new NGSILDSource(9001))
+```
+
+-   Parse the received data.
+
+```scala
+val processedDataStream = eventStream.
+    .flatMap(event => event.entities)
+    // ...processing
+```
+
+The received data is a DataStream of objects of the class **`NgsiEvent`**. This class has the following attributes:
+
+-   **`creationTime`**: Timestamp of arrival.
+
+-   **`service`**: FIWARE service extracted from the HTTP headers.
+
+-   **`servicePath`**: FIWARE service path extracted from the HTTP headers.
+
+-   **`entities`**: Sequence of entites included in the message. Each entity has the following attributes:
+
+    -   **`id`**: Identifier of the entity.
+
+    -   **`type`**: Node type.
+
+    -   **`attrs`**: Map of attributes in which the key is the attribute name and the value is an object with the
+        following properties:
+
+        -   **`type`**: Type of value (Float, Int,...).
+
+        -   **`value`**: Value of the attribute.
+
+    -   **`@context`**: Map of terms to URIs providing an unambiguous definition.
+    
+    
 ### OrionSink
 
 -   Import dependency.