Update docs

Author: m4theushw

docs/Fields.md

@@ -36,6 +36,7 @@ All field components accept the following attributes:
 * `formClassName`: A class name (usually generated by JSS) to customize the look and feel of the field container when it is used inside `<SimpleForm>` or `<TabbedForm>`.
 * `addLabel`: Defines the visibility of the label when the field is not in a `Datagrid`. Default value is `true`.
 * `textAlign`: Defines the text alignment inside a cell. Supports `left` (the default) and `right`.
+* `emptyText`: Defines a text to be shown when a field has no value.
 
 {% raw %}
 ```jsx