update installation docs

[daniloab]

hi, thanks for the repository and docs. Also, I see some improvements to aggregate to the amazing job.

I guess I will have a free time into the next days and I can send a pull request doing these items below. Until there if someone can feel free to do it:

• add the same topics from npm to yarn
• upgrade webpack section to the actual one from example
  update from

module.exports = {
  webpack: (config, { isServer, webpack }) => {
    if (!isServer) {
      // Ensures no server modules are included on the client.
      config.plugins.push(new webpack.IgnorePlugin(/lib\/server/));
    }

    return config;
  },
};

to https://github.com/RevereCRE/relay-nextjs/blob/main/example/next.config.js

module.exports = {
  webpack: (config, { isServer, webpack }) => {
    if (!isServer) {
      // Ensures no server modules are included on the client.
      config.plugins.push(new webpack.IgnorePlugin(/lib\/server/));
    }

    return config;
  },
};

• file names from relay server environments
• https://github.com/RevereCRE/relay-nextjs/blob/main/example/src/lib/relay_client_environment.ts