Update pull_request_template.md

Author: zfergus

.github/pull_request_template.md

@@ -9,6 +9,7 @@ Fixes # (issue)
 Please delete options that are not relevant.
 
 - [ ] Bug fix (non-breaking change which fixes an issue)
+- [ ] Enhancement (non-breaking change which improves existing functionality)
 - [ ] New feature (non-breaking change which adds functionality)
 - [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
 - [ ] This change requires a documentation update