persisters: document how to use a custom persister

[sckott]

notes:

• usage in ruby vcr https://github.com/vcr/vcr/search?q=persist_with
• vcr ruby persisters docs https://relishapp.com/vcr/vcr/v/2-2-4/docs/cassettes/cassette-persistence
• a gem providing a custom persister https://github.com/everypolitician/vcr-archive
• vcr ruby only provides file system persistence, so also stick with only providing that by default
• a sqlite example would make most sense, and easiest to for a user to play around with