PR template for React Native SDK (SiftScience#33)

PR template

Author: viaskal-sift

.github/pull_request_template.md

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+## Purpose
+
+## Summary
+
+## Testing
+
+## Checklist
+- [ ] The change was thoroughly tested manually
+- [ ] The change was covered with unit tests
+- [ ] The change was tested with the integration example
+- [ ] The version refers to the latest stable version of sift-android
+- [ ] The version refers to the latest stable version of sift-ios
+- [ ] Necessary changes were made in the integration example (if applicable)
+- [ ] New functionality is reflected in README (if applicable)