Serial json (#3942)

* add value_template option

* Set type to template

* Remove tab

source/_components/sensor.serial.markdown

@@ -44,6 +44,10 @@ sensor:
   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 %}