updated doc info

src/main/java/org/gcube/social_networking/socialnetworking/model/beans/catalogue/CatalogueEvent.java

@@ -24,19 +24,22 @@ public class CatalogueEvent {
 	protected String[] idsToNotify;	
 
 	/**
-	 * the item identifier or name
+	 * the item identifier or name (it is used in the subject of the email)
 	 */
 	@JsonProperty("itemId")
+	@NotNull(message="itemId cannot be missing")
 	protected String itemId;
 	/**
-	 * the text that you want to write in the notification (text/plain)
+	 * the text that you want to write in the notification (text/plain format)
 	 */
 	@JsonProperty("notifyText")
+	@NotNull(message="notifyText cannot be missing")
 	protected String notifyText;
 	/**
 	 * the URL to redirect when clicking on the notification
 	 */
 	@JsonProperty("itemURL")
+	@NotNull(message="itemURL cannot be missing")
 	protected URL itemURL;
 
 	/**