Document the new /summary endpoint for sensors.

docs/guide/assets/sensors.md

@@ -105,6 +105,32 @@ multiple requests should be made using the `offset` parameter. Setting `limit`
 to a value greater than the max will result in a response code of
 `422 Unprocessable Entity: exceeds max limit of 100000`.
 
+**Data summary info**
+
+It's also possible to retrieve summary information about the data stored for a
+particular sensor by adding `/summary` to the end of the sensor's URL. This will
+list each unique `name` used in the data, along with the total `count` of values
+and the `first` and `last` timestamp for each. For example:
+
+`https://myfarm.farmos.net/farm/sensor/listener/xxxxxx/summary?private_key=yyyyyy`
+
+Example JSON response:
+
+```
+{
+    "humid": {
+        "count": 100,
+        "first": "1541117473",
+        "last": "1594911898"
+    },
+    "temp": {
+        "count": 100,
+        "first":"1541117473",
+        "last":"1594911898"
+    }
+}
+```
+
 **Allowing public API read access**
 
 Data in farmOS is private by default. A private key is required to push data to