Add description to PULL_REQUEST_TEMPLATE (#120)

.github/PULL_REQUEST_TEMPLATE.md

@@ -24,18 +24,24 @@
 
 ## Pre-Review Checklist for the PR Author
 
+**PR Author should check the checkboxes below when creating the PR.**
+
 - [ ] Code follows [coding guidelines][coding-guidelines]
 - [ ] Assign PR to reviewer
 
 ## Checklist for the PR Reviewer
 
+**Reviewers should check the checkboxes below before approval.**
+
 - [ ] Commits are properly organized and messages are according to the guideline
 - [ ] Code follows [coding guidelines][coding-guidelines]
 - [ ] (Optional) Unit tests have been written for new behavior
 - [ ] PR title describes the changes
 
 ## Post-Review Checklist for the PR Author
 
+**PR Author should check the checkboxes below before merging.**
+
 - [ ] All open points are addressed and tracked via issues or tickets
 - [ ] Write [release notes][release-notes]
 - [ ] (When added something to `.repos`) Check if proper access rights are set