Add documentation for binary sensor map sum type(#2123)

Co-authored-by: Jesse Hills <3060199+jesserockz@users.noreply.github.com>

components/sensor/binary_sensor_map.rst

@@ -13,7 +13,7 @@ This sensor is **mostly used for touch** devices but could be used for any ``bin
 Add your binary sensors as ``channels`` to the binary sensor map. The binary sensor map then publishes a value depending
 on the type of the binary sensor map and the values specified with each channel.
 
-This platform currently supports only one measurement type: ``GROUP``, but others might get added later.
+This platform currently supports two measurement types: ``GROUP`` and ``SUM``, and others might get added later.
 You need to specify which type of mapping you want with the ``type:`` configuration value:
 
 - ``GROUP`` Each channel has its own value. The sensor publishes the average value of all active
@@ -47,6 +47,45 @@ You need to specify which type of mapping you want with the ``type:`` configurat
         channel: 0
         id: touchkey0
       # ...
+      
+- ``SUM`` Each channel has its own value. The sensor publishes the sum of all active
+  binary sensors values.
+
+.. code-block:: yaml
+
+    # Example configuration entry
+    sensor:
+      - platform: binary_sensor_map
+        id: group_0
+        name: 'Group Map 0'
+        type: sum
+        channels:
+          - binary_sensor: bit0
+            value: 1
+          - binary_sensor: bit1
+            value: 2
+          - binary_sensor: bit2
+            value: 4
+          - binary_sensor: bit3
+            value: 8
+
+    binary_sensor:
+      - platform: gpio
+        pin: 4
+        id: bit0
+
+      - platform: gpio
+        pin: 5
+        id: bit1
+
+      - platform: gpio
+        pin: 6
+        id: bit2
+
+      - platform: gpio
+        pin: 7
+        id: bit3
+      # ...
 
 Configuration variables:
 ------------------------