The skeldoc package lets you produce placeholder elements for documents under development, similar to the skeleton screens used while loading contents in many applications and websites. It also has a mechanism for attaching explanatory endnotes to these placeholders, or to anything else in your document. For an example of use, along with some explanations, see the file skeldoc.pdf, or its source.
The package is written by Magnus Lie Hetland and is released under the MIT license.