Readme.md. Mention :map field type

Author: andrykonchin

README.md

@@ -121,7 +121,8 @@ These fields will not change an existing table: so specifying a new read_capacit
 You'll have to define all the fields on the model and the data type of each field. Every field on the object must be included here; if you miss any they'll be completely bypassed during DynamoDB's initialization and will not appear on the model objects.
 
 By default, fields are assumed to be of type `:string`. Other built-in types are
-`:integer`, `:number`, `:set`, `:array`, `:datetime`, `date`, `:boolean`, `:raw` and `:serialized`.
+`:integer`, `:number`, `:set`, `:array`, `:map`, `:datetime`, `date`, `:boolean`, `:raw` and `:serialized`.
+`array` and `map` match List and Map DynamoDB types respectively.
 `raw` type means you can store Ruby Array, Hash, String and numbers.
 If built-in types do not suit you, you can use a custom field type represented by an arbitrary class, provided that the class supports a compatible serialization interface.
 The primary use case for using a custom field type is to represent your business logic with high-level types, while ensuring portability or backward-compatibility of the serialized representation.