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
+-->
+
+## Release Notes
+
+_(fill-in or delete this section)_
+
+<!--
+  If this PR makes user facing changes, please describe them here.
+  This description will be copied into the release notes, whenever
+  the next version is released. Keep this section short, and
+  focus on high level changes.
+-->
+
+## 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.
+  Include anything here that you didn't include in *Release Notes*
+  above, such as changes to CI or changes to internal methods.
+-->
+
+## 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?
+-->