Add the context key documentation

Author: Simperfit

serializer/encoders.rst

@@ -36,6 +36,13 @@ The ``JsonEncoder``
 The ``JsonEncoder`` encodes to and decodes from JSON strings, based on the PHP
 :phpfunction:`json_encode` and :phpfunction:`json_decode` functions.
 
+The ``CsvEncoder``
+~~~~~~~~~~~~~~~~~~~
+
+The ``CsvEncoder`` encodes to and decodes from CSV.
+
+You can pass the context key ``as_collection`` in order to have the results always as a collection.
+
 The ``XmlEncoder``
 ~~~~~~~~~~~~~~~~~~
 
@@ -64,6 +71,8 @@ Be aware that this encoder will consider keys beginning with ``@`` as attributes
     //     <foo bar="value" />
     // </response>
 
+You can pass the context key ``as_collection`` in order to have the results always as a collection.
+
 The ``YamlEncoder``
 ~~~~~~~~~~~~~~~~~~~