Add pull request template

.github/PULL_REQUEST_TEMPLATE/default.md

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+Fixes # .
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+Changes proposed in this pull request:
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+After submitting your PR, CI checks will run and @tiny-tim-bot will check for your CLA signature.
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+For nontrivial changes, we will conduct both a design review and a code review. Please tag a core team member for code
+review, or leave blank and we will find someone for you!
+
+- [ ] Design Review (@jcampbell)
+- [ ] Code Review
+
+Thank you for submitting!