Add mdbook-extra-watch-dirs

This helps when using `mdbook serve` so that mdbook will automatically
reload whenever a file changes.

This doesn't handle adding new files because that requires running
`generate-book.py`, but that should be fine.

Author: ehuss

book.toml

@@ -12,3 +12,6 @@ heading-split-level = 0
 
 [output.html.playground]
 runnable = false
+
+[build]
+extra-watch-dirs = ["text"]