chore: add note about providing a reference in PR template (#2161)

closes #2110

Author: pano9000

.github/pull_request_template.md

@@ -5,8 +5,11 @@ feat(validatorName): brief title of what has been done
 
 <!--- briefly describe what you have done in this PR --->
 
+<!--- provide some (credible) references showing the structure of the data to be validated, if applicable --->
+
 ## Checklist
 
 - [ ] PR contains only changes related; no stray files, etc.
 - [ ] README updated (where applicable)
 - [ ] Tests written (where applicable)
+- [ ] References provided in PR (where applicable)