add note on Symfony SE forks for bug reports
minor #4072 [Contributing][Code] add note on Symfony SE forks for bug reports (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[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.

Commits
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ebf1415 add note on Symfony SE forks for bug reports