Note that the manual was wrong when the game came out, not just as a result of a patch.
I also read readmes, but they are usually useless as a resource to users. When I want to know fact X, I can either:
a) look it up in the manual;
look it up in the manual, then check EVERY LINE OF EVERY README to see if it changed.
In computer science, we use hashtables (O(1)) rather than forcing a CPU to look at every element to find what it wants (O(n)). Why should the user be treated worse than a CPU?
For example, look at Master of Magic. It has - bar none - the best 'manual set' I've ever seen. There's a complete manual and complete spell manual.
However... you can never rely on them without also consulting the readmes, as there are a LOT of changes in the final version. I still rate MoM #1 in terms of manuals, but that's just because they had the best initial manual, and nobody else updates their manuals either. And furthermore, you can't update a hard copy, so they're exempted.