Add pull request template

Author: timcharper

PULL_REQUEST_TEMPLATE.md

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+# Pull Request Checklist
+
+Please confirm that you've done the following when opening a new pull request:
+
+  - [ ] For fixes and other improvements, please reference the GitHub issue that your change addresses.
+  - [ ] For fixes, optimizations and new features, please add an entry to the CHANGES.md file.
+  - [ ] Run mvn compile before committing, so that the auto-code formatter will format your changes consistently with the rest of the project.
+