Explain how to provide a stack trace

Author: Grégoire Paris

contributing/code/bugs.rst

@@ -32,6 +32,12 @@ If your problem definitely looks like a bug, report it using the official bug
 * Give as much detail as possible about your environment (OS, PHP version,
   Symfony version, enabled extensions, ...);
 
+* If you want to provide a stack trace, be sure to provide the plain text
+  version, which should appear at the bottom of the page. *Do not* provide it
+  as a screenshot, since search engines will not be able to index the text
+  inside them. If the stack trace is long, consider enclosing it in a
+  ``<details>`` html tag.
+
 * *(optional)* Attach a :doc:`patch <patches>`.
 
 .. _`Stack Overflow`: http://stackoverflow.com/questions/tagged/symfony2