Add PR template

.github/pull_request_template.md

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+# Zod Pull Request
+
+**IMPORTANT:** Development of the next major version of Zod (`v4`) is currently ongoing. If your PR implements new functionality, it should target the `v4` branch, NOT the `master` branch. (If it's a bugfix, the `master` branch is fine.)
+
+## Overview
+
+Thank you for your contribution to our project! Before submitting your pull request, please ensure the following:
+
+- [ ] Your code changes are well-documented.
+- [ ] You have tested your changes.
+- [ ] You have updated any relevant documentation.