Add pull request template.

Author: ghengeveld

.github/pull_request_template.md

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+# Description
+
+Describe the feature / improvement / bugfix you did. This will be in the release notes.
+Please refer to an issue number if there is one.
+
+## Breaking changes
+
+Does this include any (potentially) breaking API changes?
+
+# Checklist
+
+Make sure you check all the boxes. You can omit items that are not applicable.
+
+- [ ] Implementation for both `<Async>` and `useAsync()`
+- [ ] Added / updated the unit tests
+- [ ] Added / updated the documentation
+- [ ] Updated the PropTypes
+- [ ] Updated the TypeScript type definitions