Create pull_request_template.md (letta-ai#368)

* Create pull_request_template.md

.github/pull_request_template.md

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+**Please describe the purpose of this pull request.**
+Is it to add a new feature? Is it to fix a bug?
+
+**How to test**
+How can we test your PR during review? What commands should we run? What outcomes should we expect?
+
+**Have you tested this PR?**
+Have you tested the latest commit on the PR? If so please provide outputs from your tests.
+
+**Related issues or PRs**
+Please link any related GitHub [issues](https://github.com/cpacker/MemGPT/issues) or [PRs](https://github.com/cpacker/MemGPT/pulls).
+
+**Is your PR over 500 lines of code?**
+If so, please break up your PR into multiple smaller PRs so that we can review them quickly, or provide justification for its length.
+
+**Additional context**
+Add any other context or screenshots about the PR here.