Bind helper RFC changes

[Windvis]

Since the v3 alfa release the bind helper RFC has been changed. The helper has been renamed to fn and no longer provides any context binding functionality.

Since a polyfill for the fn helper already exists and it is part of Ember since 3.11 I think it might be better to remove the deprecation notice in the readme?

Having a bind helper can still be useful for use cases that the @action decorator + fn don't cover. For ex. Calling methods on third party classes you don't control.

[Serabe]

Fixed. Thank you!