Update ISSUE_TEMPLATE.md

Added example plunker, thanks to @nikmartin in #433 here is a demo plunker you can use as a starting point
https://plnkr.co/edit/C9mM5feGGZ0N7povA84G?p=preview

Author: Anthropic

.github/ISSUE_TEMPLATE.md

@@ -15,6 +15,10 @@ I expected ...
 It actually ...
 
 #### Gist/Plunker/Demo
+<!--
+Here is a demo plunker you can use as a starting point
+https://plnkr.co/edit/C9mM5feGGZ0N7povA84G?p=preview
+-->
 [Description](url)
 
 #### Related issues