add a section about global actions for features

The Readme so far is a bit confusing about activation of a feature for all users. It mainly explains how to do it using groups yet using ```activate_group(:feature_a, :all)``` does not cause ```active?(:feature_a)``` to be true.

So this patch rewords a bit the "Feature is broken" section by describing the 3 actions (activate, deactivate, check) from the global point of view.

Follow up : is there a way to check that a feature is active for a specific group ?