Document @ProtoAdaptor for marshalling of external classes

Protostream 4.4.0.Alpha4 added support for automatically generating marshallers for external classes via the @ProtoAdaptor field. This greatly simplifies how a user can marshall third party classes, as it's no longer necessary to implement the MessageMarshaller interface and explicitly create a SerializationContextInitializer implementation.

Process for using @ProtoAdaptor
1. Create an adaptor class and add the @ProtoAdaptor annotation

@ProtoAdapter(UUID.class)
public class UUIDAdapter {

   @ProtoFactory
   UUID create(Long mostSigBitsFixed, Long leastSigBitsFixed) {
         return new UUID(mostSigBitsFixed, leastSigBitsFixed);
   }

   @ProtoField(number = 1, type = Type.FIXED64, defaultValue = "0")
   Long getMostSigBitsFixed(UUID uuid) {
      return uuid.getMostSignificantBits();
   }

   @ProtoField(number = 2, type = Type.FIXED64, defaultValue = "0")
   Long getLeastSigBitsFixed(UUID uuid) {
      return uuid.getLeastSignificantBits();
   }
}

2. Register UUIDAdapter with your SerialiazationContextInitializer interface (LibraryInitializer in the docs examples).