Add Contribution Guide

The gematik Open Source Program Office (OSPO) strongly advises to
publish and maintain contribution guides for each gematik owned
repository. This change provides the contribution guide for the
'api-telematik' repo.

CONTRIBUTING.md

@@ -0,0 +1,44 @@
+# Welcome to the gematik contributing guide <!-- omit in toc -->
+
+Thank you for investing your time in contributing to our projects!
+
+In this guide you will get an overview how you can contribute to our projects by opening an issue, creating, reviewing and merging a pull request.
+
+Please also read our [Code of Conduct](./CODE_OF_CONDUCT.md) to understand how to keep our community approachable and respectable.
+
+Use the table of contents icon on the top left corner of this document to get to a specific section of this guide quickly.
+
+## Reporting a security vulnerability
+
+Please do not report vulnerabilities and security incidents as GitHub issues. Please contact us by sending an E-Mail to TODO or report them using the contact form at https://fachportal.gematik.de/kontaktformular.
+
+## New contributor guide
+
+To get an overview of the project, read the [README](./README.md).
+
+## Getting started
+
+### Commit your update
+
+Commit the changes once you are happy with them.
+
+### Pull Request Process
+
+- When you're finished with the changes, create a pull request, also known as a PR.
+- Fill the pull request template so that we can review your PR. This template helps reviewers to understand your changes as well as the purpose of your pull request.
+- Don't forget to [link the PR to the issue](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue) if you are solving one.
+- Update the [README.md](./README.md), the documentation and the [ReleaseRotes.md](./ReleaseNotes.md) with all details to document the changes made by this PR.
+- Increase the version numbers in any necessary files and the [readme](./README.md) to the new version that this
+  Pull Request would represent. The versioning scheme we use is [SemVer](http://semver.org/).
+- Once you submit your PR, a project team member will review your proposal. We may ask questions or request additional information.
+- We may ask for changes to be made before a PR can be merged, either using [suggested changes](https://docs.github.com/en/github/collaborating-with-issues-and-pull-requests/incorporating-feedback-in-your-pull-request)
+  or pull request comments. You can apply suggested changes directly through the UI. You can make any other changes in your fork, then commit them to your branch.
+- As you update your PR and apply changes, mark each conversation as [resolved](https://docs.github.com/en/github/collaborating-with-issues-and-pull-requests/commenting-on-a-pull-request#resolving-conversations).
+- You may merge the Pull Request in once you have the sign-off of two other developers, or if you
+  do not have permission to do that, you may request the second reviewer to merge it for you.
+
+### Your PR is merged!
+
+Congratulations :tada::tada: The gematik team thanks you :sparkles:.
+
+Once your PR is merged, your contributions will be publicly visible on the [gematik github page](https://github.com/gematik/).