chore: add PR templates (#20)

.github/PULL_REQUEST_TEMPLATE/bugfix.md

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+**IMPORTANT**:
+
+Please do not create a Pull Request without creating an issue first.
+A similar PR may already be submitted!
+Please search among the [Pull request](../) before creating one.
+
+For more information, see the `CONTRIBUTING` guide.
+
+## Summary
+
+This PR fixes the following **problems**:
+
+- Bug 1
+- Bug 2
+
+## Testing
+
+Please describe how the fix can be tested
+
+## Related isssues
+
+Put _closes #XXXX_ in your comment to auto-close the issue that your PR implements.

.github/PULL_REQUEST_TEMPLATE/feature_request.md

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+**IMPORTANT**:
+
+Please do not create a Pull Request without creating an issue first.
+A similar PR may already be submitted!
+Please search among the [Pull request](../) before creating one.
+
+For more information, see the `CONTRIBUTING` guide.
+
+## Summary
+
+This PR implements / improves the following **features**:
+
+- Feature 1
+- Feature 2
+
+**Breaking changes:**
+
+- Breaking change 1
+- Breaking change 2
+
+## Related isssues
+
+Put _closes #XXXX_ in your comment to auto-close the issue that your PR implements.