Create pull_request_template.md

Author: mishmosh

.github/pull_request_template.md

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+<!-- Thanks for your contribution! Please review the contributing guidelines before submitting: https://codeforphilly.github.io/chime/CONTRIBUTING.html -->
+
+### Summary
+- This PR affects {Operations | Deployment | Model}.
+- Fixes #(relevant issue number or numbers).
+
+### Description
+- Short description of problem & solution, for copying into changelog. (1-3 sentences, depending on scope of change)