pr-template: Remove unnecessary references section and add instructio…

…ns for the PR title. (y-scope#438)

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,9 +1,14 @@
-# References
-<!-- Any issues or pull requests relevant to this pull request -->
+<!--
+Set the PR title to a meaningful commit message in imperative form. E.g.:
+
+clp-s: Don't add implicit wildcards ('*') at the beginning and the end of a query (fixes #390).
+-->
 
 # Description
-<!-- Describe what this request will change/fix and provide any details necessary for reviewers -->
+<!-- Describe what this request will change/fix and provide any details necessary for reviewers. -->
+
+
 
 # Validation performed
-<!-- What tests and validation you performed on the change -->
+<!-- Describe what tests and validation you performed on the change. -->