Create PULL_REQUEST_TEMPLATE.md

Author: alexellis

.github/PULL_REQUEST_TEMPLATE.md

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+## Description
+
+
+## How Has This Been Tested?
+<!--- Please describe in detail how you tested your changes. -->
+<!--- Include details of your testing environment, and the tests you ran to -->
+<!--- see how your change affects other areas of the code, etc. -->
+
+
+## How are existing users impacted? What migration steps/scripts do we need?
+
+
+## Checklist:
+
+I have:
+
+- [ ] updated the documentation and/or roadmap (if required)
+- [ ] read the [CONTRIBUTION](https://github.com/openfaas/faas/blob/master/CONTRIBUTING.md) guide
+- [ ] signed-off my commits with `git commit -s`
+- [ ] added unit tests