Create PULL_REQUEST_TEMPLATE.md

Author: VKin-Arduino

PULL_REQUEST_TEMPLATE.md

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+<!-- Thanks for contributing to Science Journal iOS! Before you submit your pull request, please make sure to check the following boxes by putting an x in the [ ] (don't: [x ], [ x], do: [x]) -->
+
+### Checklist
+- [ ] All new and existing tests pass
+- [ ] I've read the [Contribution Guidelines](https://github.com/google/science-journal-ios/blob/master/CONTRIBUTING.md)
+- [ ] I've read [Change Limitations](https://github.com/google/science-journal-ios/blob/master/CHANGE_LIMITATIONS.md)
+
+### Motivation and Context
+<!-- Why is this change required? What problem does it solve? -->
+<!-- If it fixes an open issue, please link to the issue here. -->
+
+### Description
+<!-- Describe your changes in detail -->
+<!-- Please describe in detail how you tested your changes. -->