Link live demo from main River README (#842)

I was looking for the live demo this morning. You can get to it by
clicking through to "River UI" and then clicking live demo from there,
but it probably doesn't hurt to have a direct link to it from the main
README so it's one fewer hop to get there (and more people might see
it).

Author: brandur

docs/README.md

@@ -2,7 +2,8 @@
 
 River is a robust high-performance job processing system for Go and Postgres.
 
-See [homepage], [docs], and [godoc], as well as the [River UI].
+See [homepage], [docs], and [godoc], as well as the [River UI][riverui] and [its
+live demo][riveruidemo].
 
 Being built for Postgres, River encourages the use of the same database for
 application data and job queue. By enqueueing jobs transactionally along with
@@ -196,7 +197,6 @@ Thank you for driving the software ecosystem forward.
 [Error and panic handling]: https://riverqueue.com/docs/error-handling
 [Multiple queues]: https://riverqueue.com/docs/multiple-queues
 [Periodic and cron jobs]: https://riverqueue.com/docs/periodic-jobs
-[River UI]: https://github.com/riverqueue/riverui
 [Scheduled jobs]: https://riverqueue.com/docs/scheduled-jobs
 [Snoozing jobs]: https://riverqueue.com/docs/snoozing-jobs
 [Subscriptions]: https://riverqueue.com/docs/subscriptions
@@ -213,4 +213,5 @@ Thank you for driving the software ecosystem forward.
 [homepage]: https://riverqueue.com
 [maintenance services]: https://riverqueue.com/docs/maintenance-services
 [riverui]: https://github.com/riverqueue/riverui
+[riveruidemo]: https://ui.riverqueue.com
 [transactional enqueueing]: https://riverqueue.com/docs/transactional-enqueueing