minor #17997 Updated all the README files (javiereguiluz)

This PR was squashed before being merged into the 2.3 branch (closes #17997).

Discussion
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Updated all the README files

| Q             | A
| ------------- | ---
| Branch        | 2.3
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Related to #17995.

Commits
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2e81248 Updated all the README files

Author: fabpot

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)