implemented LinkEnricher registration via Enrich annotation

scm-core/src/main/java/sonia/scm/api/v2/resources/LinkEnricher.java

@@ -1,11 +1,18 @@
 package sonia.scm.api.v2.resources;
 
+import sonia.scm.plugin.ExtensionPoint;
+
 /**
  * A {@link LinkEnricher} can be used to append hateoas links to a specific json response.
+ * To register an enricher use the {@link Enrich} annotation or the {@link LinkEnricherRegistry} which is available
+ * via injection.
+ *
+ * <b>Warning:</b> enrichers are always registered as singletons.
  *
  * @author Sebastian Sdorra
  * @since 2.0.0
  */
+@ExtensionPoint
 @FunctionalInterface
 public interface LinkEnricher {