Explain how to provide a stack trace

Author: Grégoire Paris

contributing/code/bugs.rst

@@ -33,6 +33,15 @@ 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 you got on an html page, 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. Same goes for errors encountered in a
+  terminal, do not take a screenshot, but learn how to copy/paste from it. If
+  the stack trace is long, consider enclosing it in a ``<details>`` html tag.
+  Be wary that stack traces may contain sensible information, and if it is the
+  case, be sure to redact them prior to posting your stack trace.
+
 * *(optional)* Attach a :doc:`patch <patches>`.
 
 .. _`Stack Overflow`: http://stackoverflow.com/questions/tagged/symfony2