Update PULL_REQUEST_TEMPLATE.md

.github/PULL_REQUEST_TEMPLATE.md

@@ -6,6 +6,8 @@
 ## Motivation and Context
 <!--- Why is this change required? What problem does it solve? -->
 <!--- If it fixes an open issue, please link to the issue here. -->
+- [ ] I have raised an issue to propose this change ([required](https://github.com/alexellis/faas/blob/master/CONTRIBUTING.md))
+
 
 ## How Has This Been Tested?
 <!--- Please describe in detail how you tested your changes. -->