CPS-378: Add Pull Request template

.github/PULL_REQUEST_TEMPLATE.md

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+## Overview
+_A brief description of the pull request. Try to keep it non-technical._
+
+## Before
+_The current status. Please provide screenshots or gifs ([LICEcap](http://www.cockos.com/licecap/), [SilentCast](https://github.com/colinkeenan/silentcast)) where appropriate._
+
+## After
+_What has been changed. Please provide screenshots or gifs ([LICEcap](http://www.cockos.com/licecap/), [SilentCast](https://github.com/colinkeenan/silentcast)) where appropriate._
+
+## Technical Details
+_If the PR introduces noteworthy technical changes, please describe them here. Provide code snippets if necessary_
+
+### Core overrides
+_If the PR overrides/patches a core file, then please explain here, for each file:_
+
+1. _Which file it is_
+2. _The reason why it was overridden/patched_
+3. _What the override/patch consists of_
+
+## Comments
+_Anything else you would like the reviewer to note_