Just to say I would really like this as a feature. It's not found in most document or writing programs (with the exception of Writemonkey) but is quite common in text editors.
I think it is exactly the same as code folding, except in a text document rather than code in a text editor.
Allows working in sections in a single large document which can be much easier than working through lots of sub documents. If necessary, we could add a note in a text box to indicate exactly what had been folded away.