message: Update contributing description
diff: # Contributing
  -
  Thank you for taking your time to contribute to the SAP Cloud SDK!
  +## Project Structure
  +This project containes multiple packages, that are managed using `lerna`. Productive packages are located in the `packages` directory, test packages are located in the `test-packages` directory.
  +All dependencies of the packages are hoisted into the root project. Some of the packages are interdependent, therefore `npm install` won't work from within those packages. Run `npm install` in the root directory instead.
  +
  ## Testing
  In order to run all tests, execute:
  ```sh-session
  