Added a notice in PULL_REQUEST_TEMPLATE.md

Author: adamperkowski

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,5 +1,10 @@
 # Pull Request
 
+<!--
+Read Contributing Guidelines before opening a PR.
+https://github.com/ChrisTitusTech/linutil/blob/main/.github/CONTRIBUTING.md
+-->
+
 ## Title
 <!--[Provide a succinct and descriptive title for the pull request.]-->