It's a personal project that the author didn't expect anyone would ever look at. He's also a journalist, not a software developer. It's an accomplishment that he had the determination to complete his project, even if it is rough around the edges.

He's a journalist, so I'm sure he expected people would be looking at it. Reasons were probably more due to inexperience and lack of time.

I bet if you asked him nicely or kindly pointed out that he's missing that, he might add one.

Seems he did! https://github.com/cperryk/slatemail/blob/master/README.md

Great! States what it's, why you should not use it and what to use instead. Such an information is helpful.