Include the help book in the build

[sbeitzel]

Overview

• Add the default (com.hackingwithswift.ShipShape.help) help book as a resource for the ShipShape project.
• Add the source for the help book to the project, to make it easy to find and edit.
• Add a shell script to automate building and installing Middlemac for any contributors who want to work on the help book.
• Update the configuration shell script so that if it's run multiple times, it doesn't rewrite the developer's files.
• Update the documentation for the help book contribution process.

Rationale

Building a help book can be a cumbersome and involved task, yet not every contributor wants to engage with the help book. So, it should be straightforward for a new contributor to build the application and contribute code for core functionality without being distracted by the whole help book toolchain. At the same time, when someone adds a new feature, it should be relatively easy for them to be able to add a little documentation for that feature, which could be incorporated the next time the help book gets rebuilt.

Notes

This change builds on the changes proposed in PR #39