CONTRIBUTING.md: Update instructions

With changing the repository structure, the 
instructions on checking codestyle and tests did
become a little less clear. 
In order to avoid confusion, we update the 
instructions with a note.

Author: s-heppner

CONTRIBUTING.md

@@ -130,6 +130,10 @@ Python's builtin `unittest`. To install the required tools, use:
 pip install .[dev]
 ```
 
+> [!note]
+> The `.` denotes the current directory and needs to be the directory the `pyproject.toml` is located in.
+> Therefore, you need to run this command and the ones below in the `/sdk` directory (relative to the repository root).
+
 Running all checks:
 ```bash
 mypy basyx test