[Contributing][Code] add note on Symfony SE forks for bug reports

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets | 

As @javiereguiluz stated in symfony/symfony#11494, the way to report bugs can be improved. One of the things that could be done, is to add a note to make a bug reproducable by forking the Symfony Standard Edition with the modifications needed to reproduce a particular issue.
Be careful here. Forking the Symfony SE is not the right for all bug reports. When reporting a bug on some standalone component, providing a failing test case if much better. Forking should be used only for problems spanning multiple layers.