Add special type info to readme

Author: gapple

.editorconfig

@@ -11,6 +11,7 @@ indent_style = space
 indent_size = 4
 
 [*.md]
+indent_size = 2
 trim_trailing_whitespace = false
 
 [*.yml]

readme.md

@@ -39,4 +39,35 @@ Serializer::serializeList(iterable): string;
 Serializer::serializeDictionary(object): string;
 ```
 
+### Special Types
+`Serializer` will accept `string` or any `Stringable` object as an Item value, 
+but will throw a `SerializeException` if it contains any characters outside the
+printable ASCII range.
+
+Special Item types that should not be encoded as strings must be differentiated 
+by using a decorating class:
+
+- **Byte Sequences** (`\gapple\StructuredFields\Bytes`)  
+  A string containing binary data.  
+
+  `Serializer` and `Parser` handle base64 encoding and decoding the provided string.
+
+
+- **Display Strings** (`\gapple\StructuredFields\DisplayString`)  
+  A string that includes Unicode characters.
+
+  `Serializer` and `Parser` handle percent-encoding and decoding non-ascii characters.
+
+
+- **Tokens** (`\gapple\StructuredFields\Token`)  
+  A short textual word with a restricted character set.
+ 
+
+- **Dates** (`\gapple\StructuredFields\Date`)  
+  An integer timestamp
+
+  `Serializer` accepts any object that implements `\DateTimeInterface`.   
+  `Parser` will return a `\gapple\StructuredFields\Date` object.
+
+  
 [1]: https://www.rfc-editor.org/rfc/rfc9651.html