minor symfony#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

Author: weaverryan

contributing/code/bugs.rst

@@ -25,6 +25,10 @@ If your problem definitely looks like a bug, report it using the official bug
 * Describe the steps needed to reproduce the bug with short code examples
   (providing a unit test that illustrates the bug is best);
 
+* If the bug you experienced affects more than one layer, providing a simple
+  failing unit test may not be sufficient. In this case, please fork the
+  `Symfony Standard Edition`_ and reproduce your issue on a new branch;
+
 * Give as much detail as possible about your environment (OS, PHP version,
   Symfony version, enabled extensions, ...);
 
@@ -35,3 +39,4 @@ If your problem definitely looks like a bug, report it using the official bug
 .. _forum: http://forum.symfony-project.org/
 .. _IRC channel: irc://irc.freenode.net/symfony
 .. _tracker: https://github.com/symfony/symfony/issues
+.. _Symfony Standard Edition: https://github.com/symfony/symfony-standard/