Create a guide for adding new items

[tigleym]

This issue looks at creating documentation for adding new items to the game. This will involve the following steps:

1. Create a new Markdown file in book/src/ called "add_items.md" (or something similar). This will be where the content for adding new items will be.

2. In book/src/SUMMARY.md, update the link for "Adding new items" from "TODO.md" to "add_items.md".

3. Write about it!