github: add pull request DCO checklist

Signed-off-by: Hanlu Li <heuleehanlu@gmail.com>

.github/pull_request_template.md

@@ -0,0 +1,17 @@
+## Summary / 变更说明
+
+<!-- What does this change do and why? / 这项变更做了什么，为什么需要？ -->
+
+## Validation / 验证
+
+<!-- List the builds and tests you ran, or explain why they are not applicable.
+请列出执行过的构建和测试；如不适用，请说明原因。 -->
+
+## Checklist / 检查项
+
+- [ ] I have read `CONTRIBUTING.md`. / 我已阅读 `CONTRIBUTING.md`。
+- [ ] Every commit contains a DCO sign-off (`git commit -s`). /
+      每个提交都包含 DCO 签署（`git commit -s`）。
+- [ ] I have included relevant build or test results, or explained why they
+      are not applicable. /
+      我已提供相关构建或测试结果，或说明了不适用的原因。