Create CONTRIBUTING.md (#6) (#19)

CONTRIBUTING.md

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+# Welcome to the OnLaunch contributing guide for the iOS Client
+
+Thank you for taking your time to contribute to our project!
+
+## New contributor guide
+
+To get an overview of the project, please read the [README](https://github.com/kula-app/OnLaunch-iOS-Client/blob/main/README.md). There you can find the instructions on how to get started with the code.
+
+### Issues
+
+#### Create a new issue
+
+If you detect a problem, check if an issue already exists. If not, you can open a new issue and add suiting labels.
+
+#### Solve an issue
+
+Scan through our [existing issues](https://github.com/kula-app/OnLaunch-iOS-Client/issues) to find one that interests you. If you want you can filter the issues by labels. If you find an issue you want to work on, you are welcome to open a PR with a fix.
+
+#### Commit your changes
+
+When you are satisfied with your changes, commit them. 
+
+### Pull Request (PR)
+
+Create a pull request once you are finished.
+
+### Your PR got merged?
+
+Good job! We thank you for helping us making OnLaunch great!