Add documentation about textDirectionality prop

docs/APIReference-Editor.md

@@ -57,6 +57,18 @@ to fit it within your UI design.
 If this value is not set, text alignment will be based on the characters within
 the editor, on a per-block basis.
 
+#### textDirectionality
+```
+textDirectionality?: DraftTextDirectionality
+```
+Optionally set the overriding text directionality for this editor. The values
+include 'RTL' for right-to-left text, like Hebrew or Arabic, and 'LTR' for
+left-to-right text, like English or Spanish. This directionality will apply to
+the entire contents, regardless of default text direction for input text.
+
+If this value is not set, text directionality will be based on the characters
+within the editor, on a per-block basis.
+
 #### blockRendererFn
 ```
 blockRendererFn?: (block: ContentBlock) => ?Object