Please enhance readme w/ authN example

[pulkitsinghal]

Although it only took 5 mins to find:

if (this.auth) {
    request.setHeader("Authorization", "Basic " + new Buffer(this.auth.username + ":" + this.auth.password).toString('base64'))
}

It would still be nice to have a sample of how to specify username / password in this client on the README for quicker reference :)

var serverOptions = {
  host: process.env.ES_HOST,
  port: process.env.ES_PORT,
  secure: process.env.ES_SECURE,
  auth: {
    username: process.env.ES_USERNAME,
    password: process.env.ES_PASSWORD
  }
};

[phillro]

done