Create contributing guideline

docs/CONTRIBUTING.md

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+# Contributing to Phonebook
+
+## What should I know before I get started?
+Before you start contributing be sure that you have already taken a look at our [Documentation](https://t-systems-mms.github.io/phonebook/). There you can find all things you need to know before work with the project.
+
+### Mandatory stuff
+To provide good code quality we are using prettier to format all files the same way. Just make sure that you are using the Plugin [Prettier](https://prettier.io/) in your IDE. You can find our settings [here](https://prettier.io/). 
+
+## How can I contribute?
+### Reporting Bugs
+You can contribute by reporting bugs. For that there are some rules:
+- Before submitting please check that the same issue was not already submitted.
+- Use the [Bug-Report-Template](https://github.com/T-Systems-MMS/phonebook/issues/new?assignees=&labels=bug&template=bug_report.md&title=) and answer the questions.
+
+### Suggesting new Features
+If you have new ideas or have something that you really need in Phonebook you can submit a new feature request. For that there are some rules:
+- Before submitting please check that the same feature request was not already submitted.
+- Use the [Feature-Request-Template](https://github.com/T-Systems-MMS/phonebook/issues/new?assignees=&labels=feature&template=feature_request.md&title=) and answer the questions.
+
+### Your First Code Contribution
+Unsure where to begin contributing to Phonebook? You can start by looking for the ["good-first-issue"-label](https://github.com/T-Systems-MMS/phonebook/issues?q=is%3Aissue+is%3Aopen+label%3A%22good+first+issue%22).
+If you have questions about the issue or do not know where to start you can just ask under the issue.
+
+### Open Pull Requests
+
+### Reviewing a pull request
+
+## Styleguides
+### Git Commit Messages
+
+## Additional Notes
+### Issue and Pull request labels
+
+### Explanation of label "prio"