update bug report template

.github/ISSUE_TEMPLATE/bug_report.md

@@ -9,10 +9,12 @@ assignees: ''
 
 Have you checked closed issues? https://github.com/Textualize/textual/issues?q=is%3Aissue+is%3Aclosed
 
-Please give a brief but clear explanation of the issue.
+Please give a brief but clear explanation of the issue. If you can, include a complete working example that demonstrates the bug. **Check it can run without modifications.**
 
-What Operating System are you running on?
+It will be helpful if you run the following command and paste the results:
 
-Feel free to add screenshots and/or videos. These can be very helpful!
+```
+textual diagnose
+```
 
-If you can, include a complete working example that demonstrates the bug. Check it can run without modifications.
+Feel free to add screenshots and / or videos. These can be very helpful!