update readme to include opts.live

[cblgh]

I'm going through all of the hyper*universe and noticed that hypervision creates its swarms like hyperdiscovery(feed, {live: true}), where feed is a hypercore.

Currently the readme is lacking the live option, so I'm not entirely sure what it does; feels like it might let peers know that a feed is creating new data in realtime?

[joehand]

Ah yes, we need to document that! Hyperdiscovery passes that option to the feed.replicate() in hypercore.

The live option will keep the replication stream open after all data is downloaded. live: false is useful if you want to download the latest set of data and exit when the stream closes.