Including pm2 as a recommended option for queues on docs

[bernardwiesner]

Supervisor has a disadvantage of not handling 0 downtime deployments. The queued jobs are still processed after a restart of the queue worker, but there can be some lag and scheduled jobs could run a little later than intended. This can be a problem for time precise jobs.

pm2 has the option of rolling restart, and 0 downtime deployments. The setup is also easy and user friendly. It would be nice to have it as an alternative recommendation to supervisor on the docs.