Create contributing guideline

docs/CONTRIBUTING.md

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+# Contributing to Phonebook 
+
+## 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
+If you want to open a pull request please use our [template](will be added soon). Please also note the issue which get resolved and stuff the reviewers need to know before reviewing your code.
+
+## 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/).
+
+## Styleguides
+### Git Commit Messages
+Because of we are using [semantic release](https://github.com/semantic-release/semantic-release) it is important that the git messages, especially the merge messages, follow the [Angular commit Message Convention](https://github.com/angular/angular.js/blob/master/DEVELOPERS.md#-git-commit-guidelines).
+The messages should have the following format: `type(scope): subject`
+
+- *type* -> feat, fix, chore and docs\
+- *scope* -> The part of the program which will be affected \
+- *subject* -> Description of what has been changed\
+
+## Additional Notes
+### Issue and Pull request labels
+qualified: You can find additional information of what you need to do to solve this issue.
+
+good-first-issue: This Label represents problems with a lower complexity and a good way to start into the project.