Track your support requests for a month and you will find most of them are the same handful of questions. Documentation is the cheapest way to stop answering them one at a time.
Write the install guide for someone tired
Not for someone incompetent, for someone doing this at the end of a long day on a deadline. Numbered steps, exact commands, what they should see after each one. Every "and then configure the database" hides three steps that generate a support request.
Requirements at the very top
Versions, extensions, server settings, anything that must exist first. Put it where it is read before installation begins rather than discovered during it.
A troubleshooting section is worth more than a feature list
List the errors people actually hit, using the exact text of the error, because that is what they will search for. Under each one, the cause and the fix. This single section typically removes more requests than everything else combined.
You already know what belongs in it. It is whatever you have answered twice.
Show the common customisations
People will change the logo, the colours, the email templates and the currency. Document where those live and you turn five support requests into one paragraph. It also stops buyers editing core files, which is what creates the difficult questions later.
Screenshots of the admin, not the marketing shots
Marketing screenshots sell the item. Documentation screenshots show the actual screen the reader is looking at right now. They are different jobs.
Keep it in the zip
Not only on a hosted site. Buyers keep the download for years and may need it long after a link has moved. HTML or markdown in the package, so it works offline and cannot rot.
Update it when you update the code
Documentation describing the previous version is worse than none, because it costs the reader time before it fails them. Treat it as part of the release, not as something to do afterwards.
The return on it
An afternoon on the troubleshooting section pays for itself within weeks, and it keeps paying every month the item sells. It also shows up in your comments as fewer confused questions, which is exactly what your next buyer reads before deciding.
