Add PR template (#1469)

.github/pull_request_template.md

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+## High Level Overview of Change
+
+<!--
+Please include a summary/list of the changes.
+If too broad, please consider splitting into multiple PRs.
+If a relevant Asana task, please link it here.
+-->
+
+### Context of Change
+
+<!--
+Please include the context of a change.
+If a bug fix, when was the bug introduced? What was the behavior?
+If a new feature, why was this architecture chosen? What were the alternatives?
+If a refactor, how is this better than the previous implementation?
+
+If there is a design document for this feature, please link it here.
+-->
+
+### Type of Change
+
+<!--
+Please check relevant options, delete irrelevant ones.
+-->
+
+- [ ] Bug fix (non-breaking change which fixes an issue)
+- [ ] New feature (non-breaking change which adds functionality)
+- [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
+- [ ] Refactor (non-breaking change that only restructures code)
+- [ ] Tests (You added tests for code that already exists, or your new feature included in this PR)
+- [ ] Documentation Updates
+- [ ] Release
+
+## Before / After
+
+<!--
+If just refactoring / back-end changes, this can be just an in-English description of the change at a technical level.
+If a UI change, screenshots should be included.
+-->
+
+## Test Plan
+
+<!--
+Please describe the tests that you ran to verify your changes and provide instructions so that others can reproduce.
+-->
+
+<!--
+## Future Tasks
+For future tasks related to PR.
+-->