Add a pull request template

Author: TheGiraffe3

.github/pull_request_template.md

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+<!-- Lines in marks like this one are instructions for filling out the template. Those lines can be deleted. -->
+<!-- Lines in double curly brackets ({{}}) should be replaced as they describe. -->
+
+<!-- Choose one heading and delete the others, or add your own. -->
+**Missions**
+**Jobs**
+**Bug Fix**
+**Artwork**
+
+## Summary
+{{Describe and explain your changes. Include links to related issues.}}
+
+## Screenshots
+<!-- If any artwork was added or changed by this PR, the following must be provided. Otherwise, delete this section. -->
+
+{{Include before and after screenshots demonstrating your changes, if applicable.}}
+
+## Testing Done
+{{Describe how you tested these changes. Ensure that issues aren't introduced.}}
+
+## Artwork Checklist
+<!-- If any artwork was added or changed by this PR, the following must be provided. Otherwise, delete this section. -->
+
+ - [ ] I updated the copyright attributions, or decline to claim copyright of any assets produced or modified
+ - [ ] I created a PR to the [assets repo](https://github.com/Alrodel/endless-sky-assets) with the necessary blend assets: {{insert PR link}}