Add commit/issue alert to PR template. (#1945)

Author: jglogan

.github/pull_request_template.md

@@ -1,3 +1,8 @@
+> [!IMPORTANT]
+> All commits must be signed and verified. Pull requests containing unsigned or unverified commits cannot be built or merged. See the [GitHub documentation](https://docs.github.com/en/authentication/managing-commit-signature-verification/about-commit-signature-verification#about-commit-signature-verification) for instructions.
+>
+> For all but trivial fixes, make sure to first create a GitHub issue that concisely describes the bug or desired enhancement as justification for the change. Large PRs with no justifying issue will be closed.
+
 ## Type of Change
 - [ ] Bug fix
 - [ ] New feature