README.md in qrb-0.2.0 vs README.md in qrb-0.3.0

@@ -32,11 +32,11 @@
 
 # And try dressing that data
 puts schema.dress(data)
 ```
 
-## About this Q binding
+## ADTs with internal contracts
 
 Qrb tries to provide an idiomatic binding for ruby developers. In particular,
 it uses a simple convention-over-configuration protocol for information
 contracts. This protocol is easily described through an example. The following
 ADT definition:
@@ -69,15 +69,44 @@
   # ...
 
 end
 ```
 
+## ADTs with external contracts
+
+When the scenario above is not possible or not wanted (would require core
+extensions for instance), Qrb allows defining ADTs with external contracts.
+The following ADT definition:
+
+```ruby
+Color = .Color <rgb> {r: Byte, g: Byte, b: Byte} ColorContract
+```
+
+expected the following ruby module:
+
+```ruby
+module ColorContract
+
+  def self.dress(tuple)
+    Color.new(tuple[:r], tuple[:g], tuple[:b])
+  end
+
+  def self.undress(color)
+    { r: color.r, g: color.g, b: color.b }
+  end
+
+end
+```
+
 ## About representations
 
 The `Rep` representation function mapping Q types to ruby classes is as
 follows:
 
 ```ruby
+# Any type is represented by Ruby's Object class
+Rep(.) = Object
+
 # Builtins are represented by the corresponding ruby class
 Rep(.Builtin) = Builtin
 
 # Sub types are represented by the same representation as the super type
 Rep(SuperType( s | ... )) = Rep(SuperType)