Create GitBook.gitignore

This is the .gitignore file for a book following the GitBook format (basically a node.js project with the `_book` directory as default output folder). This format is used by more than an hundred books on GitHub

Here are some informations about GitBook:

- The open source command line utility: https://github.com/GitbookIO/gitbook
- The publishing platform: https://www.gitbook.io/

Some examples of books with more than 1000 stars on GitHub:

- https://github.com/numbbbbb/the-swift-programming-language-in-chinese
- https://github.com/SamyPesse/How-to-Make-a-Computer-Operating-System
- https://github.com/GitbookIO/javascript

Thank you :beer: