Create pull_request_template.md (#931)

* Create pull_request_template.md

* Update pull_request_template.md


Former-commit-id: c5dd003a1831a2cc96e2262bf0f3602d97a98e54

.github/pull_request_template.md

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+[Describe your changes and what you've already done to test it.]
+
+#### Check all that apply (change to `[x]`)
+- [ ] All changes meet [libigl style-guidelines](https://libigl.github.io/style-guidelines/).
+- [ ] Adds new .cpp file.
+- [ ] Adds corresponding unit test.
+- [ ] Adds corresponding python binding.
+- [ ] This is a minor change.