Add basic pull request template.

.github/pull_request_template.md

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+# Issues
+
+<!-- Link(s) to associated issue(s) -->
+
+<!-- Example -->
+
+<!-- 1. #123 -->
+
+# Implementation
+
+<!-- Key notes on implementation and design choices -->
+
+<!-- Example -->
+
+<!-- 1. `ProxyFactory` uses the Factory Design Pattern ... -->
+
+# Qualification
+
+<!-- Description of qualification efforts -->
+
+<!-- Example -->
+
+<!-- 1. Added new MATLAB test class `ProxyFactoryTest` ... -->
+
+# Future Directions
+
+<!-- Next steps and related issues -->
+
+<!-- Example -->
+
+<!-- 1. #456 -->
+
+# Notes
+
+<!-- Important notes, attributions, etc. -->
+
+<!-- Example -->
+
+<!-- 1. Thanks @<username> for your help with this pull request! -->