[DOC] Testing instruction

[gander]

In the sample code, use https://master.apis.dev.openstreetmap.org/ for the api and apiUrl keys, which will both allow you to safely test the tool in the context of a test server, and show users how to properly configure the test server . I haven't found the information anywhere that I have to provide the same address for both keys. I discovered this through experimentation.

You should also write that to test it locally, I must provide both supported addresses in the application configuration, i.e. both http://127.0.0.1:8080 and http://127.0.0.1:8080/land.html and that I must write them one after the other, because it is not written anywhere either.