Use a better version constraint in the readme

Using `@stable` is such a bad practice since it's basically the equivalent of `*`.

I've changed it to `0.1.*`. Once this package reaches 1.0.0, we can change it to `~1.0`.

`~1.0` means get any 1.x that is 1.0.0 or greater. `~1.4` means get any 1.x that is 1.4.0 or greater.

Using `~1.0` on the readme means the user can increment the minor version themselves to satisfy the minimum version they'd like to get the features they want.