I think the word preface refers to books. I have a small document that is in the form of a hierarchical outline. Before my document, I want to put a small paragraph with instructions how to consume the material. What should I title that small paragraph?
I have used "Summary", "Abstract", and "Overview" to provide the reader with a guide to the document and how it is organized. Generally, the notion of how to follow, understand, or "consume" the material is implicit in the wording of this section.
"Overview" is common in my industry. It is a word that is more explicit than "Introduction", putting the reader into the mindset that this is a summary of what the entire document is about, rather than just the first step into material.
If you want to be explicit on how the document should be read, there is nothing wrong with saying so in the section having this title (overview, summary, abstract).
How about an introduction:
3. An initial section of a book or article, which introduces the subject material.
"Preface" is the perfect word for many short documents. For instance in their book "Report Writing for readers with little time", Elling and colleagues suggest that most technical reports should have a preface. That preface should place the work in context and give a guide to reading it.
If it is just a reading guide as you said, "abstract", "synopsis" and "summary" are misleading headings. When I read one of these words I'd expect a summary of the content, not a guide to reading the document. However, if you combine the content summary with the how-to-read guide, these headings are perfectly fine.
"Guide to the document" or another informal heading like those suggested in Sam's answer might do as well. Use your creativity.
Last word: it really depends a lot on the kind of document you are writing and your audience.