Add PULL_REQUEST_TEMPLATE.md

.github/PULL_REQUEST_TEMPLATE.md

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+<!--
+  Thanks for opening a pull request! We appreciate your dedication and help!
+  Before submitting your pull request, please make sure to read our CONTRIBUTING guide.
+
+  The best contribution is always a PR, but please make sure to open an issue or discuss
+  your changes first, if you’re looking to submit a larger PR.
+
+  If this PR is already related to an issue, please reference it like so:
+  Resolves #123
+-->
+
+## Summary
+
+<!-- What's the motivation of this change? What does it solve? -->
+
+## Set of changes
+
+<!--
+  Roughly list the changes you've made and which packages are affected.
+  Leave some notes on what may be noteworthy files you've changed.
+  And lastly, please let us know if you think this is a breaking change.
+-->