Update README.md

Author: dbu

README.md

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+# This repository is no longer maintained
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+Due to lack of interest, we had to decide to discontinue this repository. 
+The CMF project focusses on the [Routing component](https://github.com/symfony-cmf/routing) and [RoutingBundle](https://github.com/symfony-cmf/routing-bundle), which are still in active use by other projects.
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+This repository will no longer be upgraded and marked as abandoned, but will be kept available for legacy projects or if somebody wants to experiment with the CMF.
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+You can contact us in the #symfony_cmf channel of the [Symfony devs slack](https://symfony.com/slack).
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 # Symfony CMF Resource Rest Bundle
 
 [![Latest Stable Version](https://poser.pugx.org/symfony-cmf/resource-rest-bundle/v/stable)](https://packagist.org/packages/symfony-cmf/resource-rest-bundle)