Add a pull request template

Author: aeisenberg

.github/pull_request_template.md

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+### Pull Request checklist
+
+- [ ] Add a change note if necessary. See [the documentation](https://github.com/github/codeql/blob/main/docs/change-notes.md).
+- [ ] If this PR makes significant changes to `.ql`, `.qll`, or `.qhelp` files, make sure that autofixes generated based on these changes are valid. See [the documentation](https://github.com/github/codeql-team/blob/main/docs/best-practices/validating-autofix-for-query-changes.md) (internal access required).
+- [ ] All new queries has appropriate `.qhelp`.
+- [ ] QL tests are added if necessary.
+- [ ] Test your changes in [DCA](https://github.com/github/codeql-dca/) (internal access required).