Serial json (home-assistant#3942)

* add value_template option

* Set type to template

* Remove tab

Author: dgomes

source/_components/sensor.serial.markdown

@@ -31,19 +31,23 @@ sensor:
 ```
 
 {% configuration %}
-  serial_port:
-    description: Local serial port where the sensor is connected and access is granted.
-    required: true
-    type: string
-  name:
-    description: Friendly name to use for the frontend. Default to "Serial sensor".
-    required: false
-    type: string
-  baudrate:
-    description: Baudrate of the serial port.
-    required: false
-    default: 9600 Bps
-    type: int 
+serial_port:
+  description: Local serial port where the sensor is connected and access is granted.
+  required: true
+  type: string
+name:
+  description: Friendly name to use for the frontend. Default to "Serial sensor".
+  required: false
+  type: string
+baudrate:
+  description: Baudrate of the serial port.
+  required: false
+  default: 9600 Bps
+  type: int
+value_template:
+  description: "Defines a [template](/docs/configuration/templating/#processing-incoming-data) to extract a value from the serial line."
+  required: false
+  type: template 
 {% endconfiguration %}