Add some information to the Readme about kicking the tires

[mtnygard]

For people who want to try out Vase but don't have a running Datomic transactor or installation. Maybe they don't even have a my.datomic.com account.

• Show how to use in-memory DB for dev & test

[ohpauleez]

This is already the default for Vase (the library) and the default used in the Vase Template. I'll make that clearer in the README.

[mtnygard]

Was there an update to the readme?

[ohpauleez]

On master now