add description about "compatibility_mode" for Packer
add description about "compatibility_mode" for Packer

ojundt/msgpack-ruby,larskanis/msgpack-ruby,ojundt/msgpack-ruby,larskanis/msgpack-ruby,ojundt/msgpack-ruby,okkez/msgpack-ruby,okkez/msgpack-ruby,kou/msgpack-ruby,msgpack/msgpack-ruby,cosmo0920/msgpack-ruby,msgpack/msgpack-ruby,larskanis/msgpack-ruby,ojundt/msgpack-ruby,kou/msgpack-ruby,kou/msgpack-ruby,okkez/msgpack-ruby,kou/msgpack-ruby,cosmo0920/msgpack-ruby,larskanis/msgpack-ruby,msgpack/msgpack-ruby,okkez/msgpack-ruby,cosmo0920/msgpack-ruby,cosmo0920/msgpack-ruby