docs: add pull_request_template.md

.github/PULL_REQUEST_TEMPLATE/pull_request_template.md

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+## Description
+
+## Impact on users using Chronovoyage
+
+A description of changes in specifications and user experience.
+
+## Summary of the code changes
+
+A description of added features, bug fixes, performance improvements, etc.
+
+## Evidence of tests (such as operation of command, added testcases, behavior of built documentation, and so on)
+
+Steps to reproduce:
+
+1. Prepare ...
+2. Migrate with ...
+3. Table ... is created
+
+Other cases ...