add a note about tests to the contr. guidelines (#545)

Author: bertiqwerty

CONTRIBUTING.md

@@ -26,6 +26,7 @@ Afterwards, you can check that the tests are successful via
 ```
 pytest tests/
 ```
+
 ## Coding Style
 We use [Ruff](https://docs.astral.sh/ruff/) for linting, sorting and formatting of our code.
 Our doc-strings are in [Google-style](https://sphinxcontrib-napoleon.readthedocs.io/en/latest/example_google.html).
@@ -43,6 +44,10 @@ in you local project root folder, if you want to use `pre-commit`.
 
 We make heavy use of [Pydantic](https://docs.pydantic.dev/) to enforce type checks during runtime. Further, we use [Pyright](https://github.com/microsoft/pyright) for static type checking. We enforce Pyright type checks in our CI/CD pipeline.
 
+## Tests
+
+If you add new functionality, make sure that it is tested properly and that it does not break existing code. Our tests run in our CI/CD pipeline. The test coverage is hidden from our Readme because it is not a very robust metric. However, you can find it in the outputs of our test-CI/CD-pipeline. See [example](https://github.com/experimental-design/bofire/actions/runs/13699899620/job/38310818934#step:5:795.).
+
 ## Documentation
 
 We use [MkDocs](https://www.mkdocs.org/) with [material theme](https://squidfunk.github.io/mkdocs-material/) and deploy our documentation to https://experimental-design.github.io/bofire/. Thereby, an API description is extracted from the doc-strings. Additionally, we have tutorials and getting-started-sections.