Better document Dart only usage

[mjablecnik]

Hello, can I use slang also without Flutter?
I am experimenting with use slang in my Jaspr project but I have this issue:

Because I didn't add slang_flutter package..

[mjablecnik]

I have found out that the configuration contains the flutter_integration option. After setting it to false, it works now. It might be good to mention this configuration in Step 1 of the setup as well. :)