Add pull request template

Based on the PR template used by xpring-eng:

https://raw.githubusercontent.com/xpring-eng/.github/master/.github/pull_request_template.md

.github/pull_request_template.md

@@ -0,0 +1,55 @@
+<!--
+This PR template helps you to write a good pull request description.
+Please feel free to include additional useful information even beyond what is requested below.
+-->
+
+## High Level Overview of Change
+
+<!--
+Please include a summary of the changes.
+This may be a direct input to the release notes.
+If too broad, please consider splitting into multiple PRs.
+If a relevant task or issue, 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 spec or design document for this feature, please link it here.
+-->
+
+### Type of Change
+
+<!--
+Please check [x] 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 relevant, use this section for an English description of the change at a technical level.
+If this change affects an API, examples should be included here.
+-->
+
+<!--
+## Test Plan
+If helpful, please describe the tests that you ran to verify your changes and provide instructions so that others can reproduce.
+This section may not be needed if your change includes thoroughly commented unit tests.
+-->
+
+<!--
+## Future Tasks
+For future tasks related to PR.
+-->