Update CONTRIBUTING.md

.github/CONTRIBUTING.md

@@ -2,6 +2,14 @@ Reporting Bugs
 ==============
 - Make sure you're using the latest release.
 - If possible, also check if the problem has already been fixed in git master.
-- Attach a log file made by running gnome-mpv on the command line like this:
-  `G_MESSAGES_DEBUG=all gnome-mpv`
 - Don't report multiple unrelated/barely related bugs in one issue.
+- Attach a log file.
+  	- You can make gnome-mpv print log messages by running
+	  `G_MESSAGES_DEBUG=all gnome-mpv`
+- If the issue involves crashing, include a backtrace in the report.
+	- You can create a backtrace by running
+	  `gdb -ex 'set pagination off' -ex run -ex bt gnome-mpv`
+	  and perform the action that triggers the crash.
+- If the issue is a regression, it would be very helpful if you could perform a
+  [bisect](https://git-scm.com/docs/git-bisect) to locate the commit that
+  introduces the regression.