Mention symfony PR

Author: ogizanagi

README.md

@@ -5,6 +5,8 @@ Symfony Form Simple Object Mapper
 [![Coveralls](https://img.shields.io/coveralls/Elao/FormSimpleObjectMapper.svg?style=flat-square)](https://coveralls.io/github/Elao/FormSimpleObjectMapper)
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+This library aims to ease immutable or value objects mapping, until a decision on https://github.com/symfony/symfony/pull/19367 is made.
+
 # Installation
 
 Soon on packagist.org...