readme: Clarify warning and remove claims of "hostility".

Hello everyone,

This is a first attempt at clarifying the language used in the readme. It is the opinion of several members of the community (me included) that its wording has helped the common misconception that "Guix HATES Nonguix users" spread far and wide, something that could not be further from the truth.

Feedback would be appreciated.

Best, Josselin