Add a chapter on different shells (& plugin managers)

There are a lot of shells out there which can be a difficult landscape for newcomers to navigate. And while bash, zsh and powershell are popular and well known, there are also many others around: https://en.wikipedia.org/wiki/Comparison_of_command_shells

Perhaps adding a chapter on the history, a small comparison and practical examples could be useful? + once you get deeper into some of these shells you might also want to look into plugin managers e.g. for zsh - zplugin, antigen, antibody, zgen, zplugin, etc.