Add a pull request template

.github/pull_request_template.md

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+<!---
+ This template provides some ideas of things to include in your PR description.
+ To start, try providing a short summary of your changes in the Title above.
+ If a section of the PR template does not apply to this PR, then delete that section.
+ -->
+
+## Motivation
+
+_(fill-in or delete this section)_
+
+<!--- 
+ What goal is this change working towards?
+ If this PR fixes one of more issues, list them here.
+ One line each, like so:
+ Fixes #123
+ Fixes #39
+-->
+
+## Changes
+
+_(fill-in or delete this section)_
+
+<!---
+ Provide a bullet pointed summary of how each file was changed.
+ Briefly explain any decisions you made with respect to the changes.
+-->
+
+## Testing
+
+_(fill-in or delete this section)_
+
+<!--- Describe how you tested this change. -->
+
+## Reviewer Guidelines
+
+_(fill-in or delete this section)_
+
+<!--
+ Is there any particular feedback you would / wouldn't like?
+ Which parts of the code should reviewers focus on?
+-->
+
+## Out of Scope / Future Work
+
+_(fill-in or delete this section)_
+
+<!--
+ Is this work closely related to any previous PRs?
+ Will there be more PRs in this series?
+-->