Update PR template to include issue resolution

Added a placeholder for issue resolution in PR template.

Author: mfranzke

.github/PULL_REQUEST_TEMPLATE.md

@@ -2,6 +2,8 @@
 
 <!-- Describe the big picture of your changes here to communicate to the maintainers why we should accept this pull request. If it fixes a bug or resolves a feature request, be sure to link to that issue. -->
 
+resolves # (issue number)
+
 ## Types of changes
 
 <!-- What types of changes does your code introduce?