Create PULL_REQUEST_TEMPLATE.md (#1946)

Author: Kludex

.github/PULL_REQUEST_TEMPLATE.md

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+<!-- Thanks for contributing to Starlette! 💚
+Given this is a project maintained by volunteers, please read this template to not waste your time, or ours! 😁 -->
+
+# Summary
+
+<!-- Write a small summary about what is happening here. -->
+
+# Checklist
+
+- [ ] I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
+- [ ] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
+- [ ] I've updated the documentation accordingly.