Create pull_request_template.md

Creating a template to use for submitting pull requests to the main Lux repository.

.github/pull_request_template.md

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+## Overview
+
+Describe the big picture of your changes here. If it fixes a bug or resolves a feature request, be sure to link to that issue.
+
+## Changes
+
+Describe the changes that your code introduces to Lux. Have you made corresponding tests and added the appropriate documentation for your changes?
+
+## Example Output
+
+Describe the expected behavior of your changes. If applicable, please include a screenshot.