issue template: add instructions for stack traces

ISSUE_TEMPLATE.md

@@ -5,7 +5,7 @@ If you are using the nvidia proprietary driver for any reason, you have two choi
 
 If `lsmod | grep nvidia | wc -l` shows anything other than zero, your bug report is not welcome here.
 
-Otherwise, please include the following four components in your bug report: sway version, debug log, configuration (if applicable), and an explanation of steps taken to reproduce the issue.
+Otherwise, please include the following four components in your bug report: sway version, debug log, configuration (if applicable), and an explanation of steps taken to reproduce the issue. If sway crashes, also include a stack trace.
 
 Obtain your version like so:
 
@@ -32,3 +32,11 @@ You should try to reproduce the issue with the default configuration. If you can
 * Configuration File:
 
 Finally, explain the steps you took in plain English to reproduce the problem below.
+
+* Stack Trace, if sway crashes:
+
+If you use systemd, you should be able to open the coredump of the most recent crash with GDB like so:
+
+    coredumpctl gdb sway
+
+And then type `bt full` to obtain the stack trace.