zhong/web depends on sinatra/base, conflicts on rack with rails

[sheldonh]

The README provides an example of modifying rails' router to mount a Zhong UI.

However, following the instructions gives a dependency error because zhong/web requires sinatra/base.

The naive user (me) adds sinatra-base to their Gemfile, at which point bundler complains that sinatra-base (1.4.0) and rails (5.1.5) have conflicting rack version requirements.

In fact, the user needs to add sinatra to their Gemfile, not sinatra-base. Probably worth clarifying in the README.