README.md: added 'Updating documents' section
README.md: added 'Updating documents' section

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