Add "Refactoring" type of change to PR template (#555)

Author: skarpovdev

.github/pull_request_template.md

@@ -10,6 +10,7 @@ Also, please tick the appropriate points in the checklist below.
 - [ ] Bug fix
 - [ ] Documentation fix
 - [ ] Tests improvement
+- [ ] Refactoring
 
 #### Checklist for all pull requests
 - [ ] The pull request has a description of the proposed change