Create pull_request_template.md

Author: st0012

.github/pull_request_template.md

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+Thanks for your Pull Request 🎉 
+
+**Please follow these instructions to help us review it more efficiently:**
+
+- Add references of related issues/PRs in the description if available.
+- If you're updating the readme file, make sure you followed [the instruction here](https://github.com/ruby/debug/blob/master/CONTRIBUTING.md#to-update-readme).
+
+## Description
+Describe your changes: