📝 Updated README.md with API limitation warnings

A new notice has been added to the README.md file, alerting users that QISsy is still in the development preview stage. It clarifies that the API's functionality is still limited, may not work as anticipated, and could break due to changes in the QIS system or unexpected input.

README.md

@@ -2,6 +2,12 @@
 
 [![Deployment to testserver](https://github.com/lennartkaden/QISsy/actions/workflows/deploy.yml/badge.svg?branch=master)](https://github.com/lennartkaden/QISsy/actions/workflows/deploy.yml)
 
+| :exclamation:  QISsy is still a dev preview! |
+|----------------------------------------------|
+The functionality of this API is still limited and may not work as expected. Please report any issues you encounter.
+The scraping process is very trivial and may break at any time due to changes in the QIS system or due to 
+unexpected input.
+
 ## Disclaimer
 
 This project is not affiliated with any University or the QIS Product by the HIS eG. It is a private project that