Docs for `Builder` should include example on how to construct an in-memory DB

[billythedummy]

Recently upgraded from 0.2 to 0.3. Had to dig through the code to figure out that Builder::new_local(":memory:") is what replaces the deprecated Database::open_in_memory() because it's not immediately obvious that :memory: is a valid path

Edit: I realize now that it's a sqlite thing but itd be nice to confirm that the option is supported