Add pull request template

.github/PULL_REQUEST_TEMPLATE.md

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+## Ready for merge?
+**YES | NO**
+
+#### What does this PR do?
+A few sentences describing the overall goals of the pull request's commits.
+Why are we making these changes? Is there more work to be done to fully
+achieve these goals?
+
+#### Helpful background context (if appropriate)
+
+#### How can a reviewer manually see the effects of these changes?
+
+#### What are the relevant tickets?
+- https://cyber.harvard.edu/projectmanagement/issues/*X*
+
+#### Screenshots (if appropriate)
+
+#### Todo:
+- [ ] Tests
+- [ ] Documentation
+- [ ] Stakeholder approval
+
+#### Requires Database Migrations?
+YES | NO
+
+#### Includes new or updated dependencies?
+YES | NO