Add pull request template

.github/pull_request_template.md

@@ -0,0 +1,15 @@
+<!-- What has been done? Why? What problem is being solved? -->
+
+
+I haven't forgotten about:
+- [ ] Tests
+- [ ] Changelog
+- [ ] Documentation
+- [ ] Commit messages comply with the [guideline](https://www.tarantool.io/en/doc/latest/dev_guide/developer_guidelines/#how-to-write-a-commit-message)
+- [ ] Cleanup the code for review. See [checklist](https://github.com/tarantool/cartridge-java/blob/master/docs/review-checklist.md)
+
+Related issues:
+<!-- Needed for #123 -->
+<!-- See also #456, #789 -->
+<!-- Part of #123 -->
+<!-- Closes #456 -->