Improve PR template

Signed-off-by: Enrico Usai <[email protected]>

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,22 @@
-*Issue #, if available:*
 
-*Description of changes:*
+### Description of changes
+* Describe *what* you're changing and *why* you're doing these changes.
+* Link to impacted open issues.
 
+### Tests
+* Describe the automated and/or manual tests executed to validate the patch.
+* Describe the added/modified tests.
 
-By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
+### References
+* Link to related PRs in other packages (i.e. cookbook, node).
+* Link to documentation useful to understand the changes.
+
+### Checklist
+- [ ] Make sure you are pointing to **the right branch** and add a label in the PR title (i.e. **2.x** vs **3.x**)
+- [ ] Check all commits' messages are clear, describing what and why vs how.
+- [ ] Make sure **to have added unit tests or integration tests** to cover the new/modified code.
+- [ ] Check if documentation is impacted by this change.
+
+Please review the [guidelines for contributing](../CONTRIBUTING.md) and [Pull Request Instructions](https://github.com/aws/aws-parallelcluster/wiki/Git-Pull-Request-Instructions).
+
+By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.