Update README to mention second-precision jobs

Lack of support for this keeps getting raised as an issue on GitHub,
despite the fact it is in fact supported:

  #103
  #67
  #26

README.md

@@ -81,6 +81,25 @@ like this `'0 22 * * 1-5 America/Chicago'`.
 
 See [rufus-scheduler documentation](https://github.com/jmettraux/rufus-scheduler#a-note-about-timezones) for more information. (note. Rufus scheduler is using Fugit under the hood, so documentation for Rufus Scheduler can help you also)
 
+
+#### Second-precision cronlines
+
+In addition to the standard 5-parameter cronline format, sidekiq-cron supports scheduling jobs with second-precision using a modified 6-parameter cronline format:
+
+`Seconds Minutes Hours Days Months DayOfWeek`
+
+For example: `"*/30 * * * * *"` would schedule a job to run every 30 seconds.
+
+Note that if you plan to schedule jobs with very low second intervals then you may need to adjust the Sidekiq default poll interval so it is lower than the interval of your jobs:
+
+```ruby
+Sidekiq.configure_server do |config|
+  config.average_scheduled_poll_interval = 2
+end
+```
+
+The default value at time of writing is 5.
+
 ### What objects/classes can be scheduled
 #### Sidekiq Worker
 In this example, we are using `HardWorker` which looks like: