Create pull_request_template.md (#61)

Added Github's PR template

.github/pull_request_template.md

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+## Description of the changes
+<!-- Why is this change required? What problem does it solve? -->
+
+<!-- This PR Fixes #issue_number -->
+
+## Type of change
+- [ ] Bug fix (non-breaking change which fixes an issue)
+- [ ] New feature (non-breaking change which adds functionality)
+- [ ] Formal verification
+- [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
+- [ ] Refactorization (non-breaking change which improves code quality)
+
+## How to test this PR?
+<!-- E.g., Run qemu-based hypervisor and then launch Linux-based confidential VM. -->