add examples to docs

[gggrafff]

I found in the kelindar source code that it is possible to customize serialization (custom GetBinaryCodec or custom MarshalBinary+UnmarshalBinary methods), but there is only one sentence about this in the documentation: Support for custom BinaryMarshaler and BinaryUnmarshaler for tighter packing control

I think we need describe this in details and with examples of code