README is unclear about persistence limitations with CRuby

[rcrogers]

https://github.com/relevance/diametric/blob/ae5fa808ae263c95eacb3ad896655e74d6f2a3b2/README.md

Diametric supports both CRuby and JRuby. When Diametric is used on CRuby, Diametric connects to Datomic's REST service. Using Datomic's REST API, Diametric creates schema/data and makes a queries to Datomic. When Diametric is used on JRuby, both Datomic's REST and Peer services are supported. The core parts of Peer service are implemented using Datomic API.
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The biggest difference between REST and Peer services is available database types. REST service can use memory database only, which means we can't save data in any file as long as using free version. While Peer service can use memory and free version of transactor, which means we can save data in a file through Datomic's transactor.

I found multiple parts of this confusing:

• "REST service can use memory database only" -- I can't find any documentation about this on the Datomic site. Are more details available?
• "can't save data in any file as long as using free version" -- what if I want to use CRuby with Datomic Pro Starter? Can I persist data to disk in that scenario?