Update PULL_REQUEST_TEMPLATE.md

Author: dario-coscia

.github/PULL_REQUEST_TEMPLATE.md

@@ -1 +1,12 @@
-Fixes #ISSUE_NUMBER
+## Description
+<!-- Provide a standalone description of changes in this PR. -->
+<!-- Reference any issues closed by this PR with "closes #1234". -->
+
+This PR fixes #ISSUE_NUMBER. <!-- Possible other comments. -->
+
+## Checklist
+
+- [ ] Code follows the project’s [Code Style Guidelines](https://github.com/mathLab/PINA/blob/master/CONTRIBUTING.md#code-style--guidelines)
+- [ ] Tests have been added or updated
+- [ ] Documentation has been updated if necessary
+- [ ] Pull request is linked to an open issue