title: Protobuf-Net ProtoMember "IsRequired" attribute usage
body: <p>What is the usage for the Protobuf-Net ProtoMember "IsRequired" attribute? </p>
  
  <p>What effect will it have if I add a new property (member) to an already serialized class in a file. If I use "IsRequired=true" will it just accept it without loading a value (since it's not there) or should I set the value to false? And if I set it to false, will the value be serialized properly?</p>