docs(pull-request): add pr template

.github/pull_request_template.md

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+## Description
+
+<!--- Describe your changes in further detail, if necessary. -->
+<!--- Please add a link to a github issue, if exists -->
+
+## Checklist
+
+<!--- This checklist serves as a tool for the developer and reviewer. Check the boxes if a criteria does not apply to you're merge request or it is resolved. -->
+
+### Documentation
+
+- [ ] Documentation is generated (if updated) (`composer docs-generator`)
+- [ ] Pull request is added to CHANGELOG.md
+
+### Coding Style
+- [ ] Request classes 
+    - [ ] follow the existing [naming conventions](https://github.com/5pm-HDH/churchtools-api/blob/master/docs/out/Requests.md) (plural for builders, singular for objects)
+- [ ] Model classes 
+  - [ ] have default values for properties and protected visibility
+  - [ ] have getters and setters for properties
+- [ ] methods with return type "array" have PHPDoc annotation with class if applicable (e.q.: `/** @return Group[] */`)
+- [ ] CodingStyle enforcement is executed (`composer php-cs-fixer`)
+
+### Test
+
+- [ ] Integration test is added (if necessary)
+- [ ] Unit test is created (if necessary)