Add a basic guide how to write tests

README.md

@@ -92,3 +92,16 @@ If you want to install this client into another project without publishing it (e
 ### Run tests
 
 To run the tests, run `poetry run pytest`.
+
+### How to write test cases
+
+There are two things to test: The models and the API functions.
+
+Lets imagine you want to write a test case for the endpoint `/api/v1/albums`
+focusing on the models for now. Since this endpoint lists the Albums, the
+correct API call is in `api/albums/albums_list.py`. Check the function called
+`_parse_response()`. The model used to parse the response is called
+`PaginatedAlbumList`, which we will run tests against. Now curl the endpoint you
+want to write tests for and put the response into `tests/data/albums.json`. Now
+we can load this json file, load it with the model and do some assertions. The
+example is available in `tests/unit/test_model_paginated_album_list.py`.