README doesn't specify what the 'database' is for TEST_DATABASE_URL

[Susurrus]

I'm working on committing to crates.io, but I don't know how to run the official tests because I don't know what "database" means in the context of the README, specifically what kind of database the TEST_DATABASE_URL should point to. Both the database type and minimum version information is missing I think.

[carols10cents]

Great suggestion! I've added some docs in #550, wdyt?

[Susurrus]

Awesome, that's very helpful. That seems clear and concise, though I didn't follow the instructions to test, but I can do that if you'd like (tomorrow I'll probably have time).