chore: update pull request template (#7)

Co-authored-by: Sourcegraph <[email protected]>

.github/pull_request_template.md

@@ -2,9 +2,10 @@
 
 A clear and concise description of what the pull request is about, i.e. what problem should be fixed? 
 
-Link to the issue: <!-- paste the issue link here -->
+Link to the issue: <!-- paste the issue link here, or put "n/a" if not applicable -->
 
 ### Checklist
 
-- [ ] I have added test cases to cover the new code.
 - [ ] I agree to follow this project's [Code of Conduct](https://golang.org/conduct) by submitting this pull request.
+- [ ] I totally understand sending a pull request has no guarantee to be merged, and the review rounds may take a long time.
+- [ ] I have added test cases to cover the new code.