docs - add jsonName example for the @alias annotation

activate-docs/spray-json.md

@@ -62,4 +62,14 @@ There are methods to create or update entities based on a json:
 def createOrUpdateEntityFromJson[E <: Entity : Manifest](json: String): E
 def createOrUpdateEntityFromJson[E <: Entity : Manifest](json: JsObject): E
 ```
-If the json has an id, the entity is updated. Otherwise, a new entity is created.
+If the json has an id, the entity is updated. Otherwise, a new entity is created.
+
+## Field alias ##
+
+The @Alias annotation allows to define the "jsonName" to be used during the field serialization.
+
+Example:
+
+```scala
+class Person(@Alias(value = "PERSON_NAME", jsonName = "name") name: String) extends Entity
+```