Updated all the README files

Author: javiereguiluz

README.md

@@ -7,8 +7,7 @@ various Symfony components.
 Resources
 ---------
 
-You can run the unit tests with the following command:
-
-    $ cd path/to/Symfony/Bridge/Doctrine/
-    $ composer install
-    $ phpunit
+  * [Contributing](https://symfony.com/doc/current/contributing/index.html)
+  * [Report issues](https://github.com/symfony/symfony/issues) and
+    [send Pull Requests](https://github.com/symfony/symfony/pulls)
+    in the [main Symfony repository](https://github.com/symfony/symfony)