update readme to show how to replicate feed

[ZhouHansen]

How to replicate hypercore, like discovery-swarm

feed.ready(function () {
  // we use the discovery as the topic
  swarm.join(feed.discoveryKey)
  swarm.on('connection', function (connection) {
    console.log('(New peer connected!)')

    // We use the pump module instead of stream.pipe(otherStream)
    // as it does stream error handling, so we do not have to do that
    // manually.

    // See below for more detail on how this work.
    pump(connection, feed.replicate({ live: true }), connection)
  })
})

[ZhouHansen]

According to #7

// {live: true} keep the replication stream open
var sw = discovery(feed, {live: true})

Hope the readme can be updated

[RangerMauve]

Feel free to submit a PR with the sort of code you'd like to see in the README. :)

[ZhouHansen]

After I red the source code, I found pass {live: true} makes no sense.