minor symfony#10369 [Serializer] Updated the list of loaded encoders (AndreiIgna)

This PR was submitted for the master branch but it was merged into the 3.4 branch instead (closes symfony#10369).

Discussion
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[Serializer] Updated the list of loaded encoders

Framework bundle preloads encoders https://github.com/symfony/framework-bundle/blob/master/Resources/config/serializer.xml#L102
but not all were added in doc page https://symfony.com/doc/master/serializer.html#adding-normalizers-and-encoders

Commits
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f4cf54a [Serializer] Updated the list of loaded encoders

Author: javiereguiluz

serializer.rst

@@ -93,6 +93,8 @@ Encoders supporting the following formats are enabled:
 
 * JSON: :class:`Symfony\\Component\\Serializer\\Encoder\\JsonEncoder`
 * XML: :class:`Symfony\\Component\\Serializer\\Encoder\\XmlEncoder`
+* CSV: :class:`Symfony\\Component\\Serializer\\Encoder\\CsvEncoder`
+* YAML: :class:`Symfony\\Component\\Serializer\\Encoder\\YamlEncoder`
 
 As well as the following normalizers: