Added documentation on fetching GeoJSON using the API

Author: rimi-itk

modules/os2forms_webform_maps/README.md

@@ -17,6 +17,55 @@ The module can be installed using the standard Drupal installation procedure.
 
 ## Fetching GeoJSON using the API
 
-``` shell
-@todo
+Assume that we have a webform with ID `my_webform` and the webform has a Map element with ID `my_map`. We can then fetch
+the data for a submission with UUID `c34d01c5-7bd9-4b15-8b01-5787959453c0` on the webform with a HTTP `GET` request (cf.
+[OS2Forms REST API](https://github.com/itk-dev/os2forms_rest_api/blob/main/README.md)):
+
+``` shell name=api-fetch-submission-data
+curl --silent --location --header 'api-key: my-api-key' http://127.0.0.1:8000/webform_rest/my_webform/submission/c34d01c5-7bd9-4b15-8b01-5787959453c0
+# {"entity":{"serial":[{"value":1}],"sid":[{"value":2}],"uuid":[{"value":"c34d01c5-7bd9-4b15-8b01-5787959453c0"}],…```
+```
+
+The result, however, contains a lot of data and we need a little more work the extract just the GeoJSON data.
+
+Using [`jq`](https://jqlang.org/) (or similar tools), we can drill down into the full submission data to extract just
+the GeoJSON data from the Map element (JSON decoding twice along the way):
+
+``` shell name=api-extract-map-element-data
+curl --silent --location --header 'api-key: my-api-key' http://127.0.0.1:8000/webform_rest/my_webform/submission/c34d01c5-7bd9-4b15-8b01-5787959453c0 | jq '.data.my_map | fromjson | .geojson | fromjson'
+```
+
+The final result will be something like
+
+``` json
+{
+  "type": "FeatureCollection",
+  "features": [
+    {
+      "type": "Feature",
+      "properties": {},
+      "geometry": {
+        "type": "LineString",
+        "coordinates": [
+          [
+            10.193674,
+            56.158929
+          ],
+          [
+            10.241758,
+            56.155296
+          ],
+          [
+            10.242788,
+            56.130048
+          ],
+          [
+            10.189209,
+            56.129283
+          ]
+        ]
+      }
+    }
+  ]
+}
 ```