chore: add pull request template for better contribution guidelines (#5057)

This PR adds a small pull request template that should make it easier
for maintainers to test new changes as they come in.

Author: Brend-Smits

.github/pull_request_template.md

@@ -0,0 +1,19 @@
+## Description
+
+<!-- Briefly describe the changes in this PR. -->
+
+## Test Plan
+
+<!--
+  Please describe how you tested these changes. This helps reviewers understand
+  the scope and confidence level of the change. Examples:
+
+  - Ran `terraform plan` against an existing deployment
+  - Added/updated unit tests (link to test file or describe coverage)
+  - Deployed to a test environment and triggered a workflow run
+  - Validated with `terraform validate` and `tflint`
+-->
+
+## Related Issues
+
+<!-- Link any related issues, e.g. Fixes #123, Closes #456 -->