Readme features invalid composer-arg `--stability=dev`

There is no composer argument `--stability=dev` (any more?).

The `create-project` help text says:
>  To setup a developer workable version you should create the project using the source
 controlled code by appending the '--prefer-source' flag. Also, it is
 advisable to install all dependencies required for development by appending the
 '--dev' flag.

But I think this is not needed to install satis?