Added docs about contributions

Author: Nyholm

README.md

@@ -24,3 +24,9 @@ $bundles(
 ```
 
 Read the documentation at [www.php-cache.com/symfony/adapter-bundle](http://www.php-cache.com/en/latest/symfony/adapter-bundle/).
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+### Contribute
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+Contributions are very welcome! Send a pull request or report any issues you find on the [issue tracker](http://issues.php-cache.com).
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