[persisted-scope] add docs for "protocol-asset" feature

[RobZuazua]

Add "protocol-asset" feature documentation to readme for Persisted Scope plugin.

many people (including myself) have to figure it out by looking at the source code

[rumitvn]

Opened #3443 to document this — added a "Persisting the asset protocol scope" section to the persisted-scope README explaining the protocol-asset feature (and clarified that only the filesystem scope is persisted by default).