Add an appendix with nice-to-know extra stuff

[MatthijsBlom]

Let's add an appendix dedicated to topics that do not fit into the main text well, but are still good to have learned about by the time one finishes the book.

This idea was inspired by #81/#137. The recently added demonstration of PatternGuards is both broken and misplaced, and I think should just be removed.

However, pattern guards are nice to know about! It'd be a shame if they weren't mentioned anywhere in the book. I don't expect there to be another spot in the book truly suitable for a discussion of pattern guards. But we don't really need one either, clearly, as the book has been doing fine without one for many years. The same probably goes for several other topics.

[ulysses4ever]

@MatthijsBlom I'm sorry for radio silence on your threads. I've been having a rough week. I'll try to reply as much as possible by the end of week.

[MatthijsBlom]

Don't worry! Do take your time, I really don't mind.