format/Messages.fbs mentions DictionaryBatches with an id but does not specify where they are referenced.
We should add a dictionary: long in Field that references the dictionary id:
We need a spec in format/Layout.md that describes the dictionary layout.
When dictionary encoded the value vector is an array of signed int32 (for consistency with variable length collection offsets).
The dictionary vector is a Vector of the type of the value. indexed by their id in the dictionary.