Adding name recommendations for allowed values

docs-gen/content/rule_set/data_entry/allowed.md

@@ -0,0 +1,37 @@
+---
+title: "Value restrictions"
+date: 2019-08-04T12:37:12+02:00
+weight: 5
+---
+
+Optionally it is possible to define an array of `allowed` values, which will restrict the usage of the data entry in the implementation of the specification.
+It is expected, that any value not mentioned in the array is considered an error and the implementation of the specification shall react accordingly.
+The datatype of the array elements is the `datatype` defined for the data entry itself.
+For `attributes` it is possible to optionally set a default value.
+
+```YAML
+SteeringWheel.Position:
+  datatype: string
+  type: attribute
+  default: FRONT_LEFT
+  allowed: [FRONT_LEFT, FRONT_RIGHT]
+  description: Position of the steering wheel on the left or right side of the vehicle.
+
+```
+
+If `allowed` is set, `min` or `max` cannot be defined.
+
+The `allowed` element is an array of values, all of which must be specified
+in a list.  Only values can be assigned to the data entry, which are
+specified in this list.
+
+The `datatype` specifier gives the type of the individual elements of the `allowed`
+list.
+
+For string values used for `default` and `allowed` statements it is recommended to start with `A-Z`
+and then use only `A-Z`, `0-9` and underscore (`_`).
+Single(`'`) and double (`"`) quotes shall be used only if necessary, and are not needed if the naming recommendation is followed.
+It is recommended not to specify a dedicated value corresponding to "unknown" or "undefined" unless there is a relevant use-case for that particular signal.
+The background is that a signal with an array of allowed values shall be handled just as any other signal.
+If e.g. the value of current speed or vehicle weight is unknown, then the vehicle shall not publish the corresponding signal.
+Similarly, for the example above, if the steering wheel position is unknown then `SteeringWheel.Position`shall not be published.