[Persistence service] Comment more on availability in README

[jamesscottbrown]

The README for the Persistence Service says: "This is also the most straightforward persistence service implementation and is not highly available."

This is potentially worrying to a user: it would be helpful to say something about whether being 'not highly available' is likely to cause problems, and what higher-availability alternatives are available (redis?).

[senakafdo]

Yes, I was planning to implement another persistence service(s) that depends on Redis or Mongo. At the moment, I cannot elaborate further since those alternatives are not yet implemented.