Docs for dummy data

dummy-data/README.md

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+# Dummy data
+
+This directory content makes possible to test custom services cards or create a demo without actually running the service. 
+The principle is simple: save a sample output of the API used in the service in a static file in this directory. The path must be identical as the service endpoint to be used seamlessly.
+
+## How to add a new services sample: 
+
+- create a directory for your service, and any sub-folder existing in the service api path. 
+- save the api output in a file named after the service endpoint.
+
+Example:
+```
+mkdir pihole
+curl http://my-pihile.me/admin/api.php -o pihole/api.php # /admin is omited because for PiHole, the implementation expect it to be in the base url (`url` or `enpoint` property)
+```