Added documentation how to custom serialize and preserve symbols
Added documentation how to custom serialize and preserve symbols

ojundt/msgpack-ruby,cosmo0920/msgpack-ruby,ojundt/msgpack-ruby,cosmo0920/msgpack-ruby,ojundt/msgpack-ruby,msgpack/msgpack-ruby,msgpack/msgpack-ruby,cosmo0920/msgpack-ruby,ojundt/msgpack-ruby,cosmo0920/msgpack-ruby,msgpack/msgpack-ruby