Update CONTRIBUTING.md

CONTRIBUTING.md

@@ -16,6 +16,7 @@ This section is split into two section:
 * We do not support Internet Explorer and Safari (Patches accepted though, except for IE < 10)
 * We do **not support the server-side encryption app** (use client side encryption instead)
 * Get the newest version of the News app
+* Make sure that it's not caused by a browser add-on by testing with all add-ons disabled
 * Clear your PHP opcode cache if you use any by restarting your webserver. This affects any version of PHP >=5.5
 * [Check if they have already been reported](https://github.com/owncloud/news/issues?state=open)
 * [Check if your problem is covered in the FAQ section](https://github.com/owncloud/news#faq) or [Updating notices](https://github.com/owncloud/news#updating-notices)