docs: add PR template (runatlantis#2718)

.github/PULL_REQUEST_TEMPLATE.md

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+## what
+
+<!--
+* Describe high-level what changed as a result of these commits (i.e. in plain-english, what do these changes mean?)
+* Use bullet points to be concise and to the point.
+-->
+
+
+## why
+
+<!--
+* Provide the justifications for the changes (e.g. business case). 
+* Describe why these changes were made (e.g. why do these commits fix the problem?)
+* Use bullet points to be concise and to the point.
+-->
+
+
+## references
+
+<!--
+* Link to any supporting github issues or helpful documentation to add some context (e.g. stackoverflow). 
+* Use `closes #123`, if this PR closes a GitHub issue `#123`
+-->
+