Update readme.txt

This one has bugged me for a while, since the previous version is blatantly untrue. This is at least a bit better.

Author: perlun

readme.txt

@@ -13,7 +13,10 @@ and register it with your client:
 var client = new RestClient();
 client.JsonSerializer = new YourCustomSerializer();
 
-The default Json*Deserializer* should be 100% compatible.
+The default Json*Deserializer* is mostly compatible, but it does not support
+all features which Json.NET has (like the ability to support a custom [JsonConverter]
+by decorating a certain property with an attribute). If you need these features, you
+must take care of the deserialization yourself to get it working.
 
 If you run into any compatibility issues with deserialization, 
-please report it to http://groups.google.com/group/restsharp
+please report it to http://groups.google.com/group/restsharp