docs: add information about approval tests

docs/development-setup.md

@@ -70,6 +70,7 @@ celery --app sketch_map_tool.tasks worker --loglevel=INFO
 
 ```bash
 mamba activate smt
+pybabel compile -d sketch_map_tool/translations
 flask --app sketch_map_tool/routes.py --debug run
 # Go to http://127.0.0.1:5000
 ```
@@ -102,6 +103,19 @@ To run all tests:
 pytest
 ```
 
+Some test are using the [Approval Testing methodology](https://approvaltests.com/).
+Approval tests capture the output (snapshot) of a piece of code and compare it
+with a previously approved version of the output.
+
+Once the output has been *approved* then as long as the output stays the same the
+test will pass. A test fails if the *received* output is not identical
+to the approved version. In this case, the difference of the received and the
+approved output is reported to the tester.
+
+In the case of the Sketch Map Tool the report takes the form of two images
+side-by-side, the uploaded sketch map with markings (input) and the resulting
+GeoJSON with the detected markings.
+
 #### Integration Tests
 
 The integration test suite utilizes the Testcontainers framework to run unique instances of Redis and Postgres for each test session. It also configures and starts Flask and Celery workers in the background.