Add pull request template for contributor guidance

Add a pull request template to guide contributors in providing necessary information.

Author: airween

.github/PULL_REQUEST_TEMPLATE.md

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+<!-- Thank you for contributing to OWASP ModSecurity, your effort is greatly appreciated -->
+<!-- Please help us by adding the information below in this PR so it aids reviewers -->
+
+## 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`
+-->