haml and slim deprecation should be reflected in --help text and README

[fwolfst]

haml and slim support were / are getting dropped as detailled and reasoned in
http://www.yukinishijima.net/2017/01/14/kaminari-1-0-0.html#the-e-haml-slim-options-for-rails-g-kaminari-views-have-been-deprecated .

However, this is not reflected by the projects README or the --help output of the rake task, leading to confusions once one finds the commits that removed the support :) .

I'll draft a PR if I find time.