Update PR Template

Author: MarkBaker

.github/PULL_REQUEST_TEMPLATE.md

@@ -3,14 +3,21 @@ This is:
 ```
 - [ ] a bugfix
 - [ ] a new feature
+- [ ] refactoring
+- [ ] additional unit tests
 ```
 
 Checklist:
 
 - [ ] Changes are covered by unit tests
+  - [ ] Changes are covered by existing unit tests
+  - [ ] New unit tests have been added
 - [ ] Code style is respected
 - [ ] Commit message explains **why** the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
 - [ ] CHANGELOG.md contains a short summary of the change
 - [ ] Documentation is updated as necessary
 
 ### Why this change is needed?
+
+Provide an explanation of why this change is needed, with links to any Issues (if appropriate).
+If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will mae it easier to track progress with this PR.