Resolve "Allow collection and circular attributes as endpoint targets"

Closes #53 (closed)

• Check, if collection and circular attributes can be specified as send endpoint targets.
• Add test cases to ensure correct handling.
• Update documentation, that they can be used.

pages/docs/using.md

@@ -244,3 +244,14 @@ assertEquals(receiverRoot.getAList(), list("1", "other"));
 // after receiving "new" on topic "some/topic/one"
 assertEquals(receiverRoot.getAList(), list("1", "other", "new"));
 ```
+
+## Using attributes as endpoint targets
+
+As described in the [DSL specification](/dsl), attributes can be used as endpoint targets.
+They can only be used in send endpoints, and the return type of the attribute must be specified in the connect specification (because aspect files are not handled completely yet).
+
+Currently, synthesized, inherited, collection, and circular attributes are supported.
+Nonterminal attributes are best used with the "legacy" notation `/Context:Type/` within the grammar.
+
+Please note, that serialization of Java collections of nonterminals is not supported, e.g., a `java.util.Set<ASTNode>`.
+Only list nodes as defined in the grammar `/Context:Type*/` are properly recognized.