Add pull request template (#731)

.github/pull_request_template.md

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+## Summary
+
+Briefly describe the changes and the goal of this PR. Make sure the PR title summarizes the changes effectively.
+
+## Changes
+
+List the major and minor changes.
+
+- Major Changes
+  - ...
+- Minor Changes
+  - ...
+
+## Motivation
+
+Why are these changes necessary? How do they improve the cookbook?
+
+## Self Review Checklist
+
+- [ ] Is the writing easily skimmable?
+  - Sections have informative titles.
+  - Key takeaways are upfront.
+  - Short paragraphs and topic sentences are used.
+- [ ] Is the writing quality high?
+  - Sentences are simple and unambiguous.
+  - Demonstrative pronouns are avoided or clearly referenced.
+  - No left-branching sentences.
+- [ ] Is the content universally helpful?
+  - Terminology is specific and avoids jargon.
+  - Provides solutions to common problems.
+  - Code examples are general and exportable.
+- [ ] Is the content consistent?
+  - Styling and formatting align with existing documentation.
+  - Consistent use of punctuation and case.
+
+**Note:** For additional guidelines on writing good documentation, check out [What Makes Documentation Good](https://cookbook.openai.com/what_makes_documentation_good).