Update pull request template

Signed-off-by: Beat Buesser <[email protected]>

Author: Beat Buesser

PULL_REQUEST_TEMPLATE.md

@@ -8,6 +8,7 @@ Fixes # (issue)
 
 Please check all relevant options.
 
+- [ ] Improvement (non-breaking)
 - [ ] Bug fix (non-breaking)
 - [ ] New feature (non-breaking)
 - [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)