Release notes and READMEs can be really important if your customer has to take special action in addition to normal procedures in order to upgrade. It also be useful to warn customers/users of any db upgrades that can automatically happen as a result of installing a newer patch. The way I see it, Release Notes and READMEs should be written for the System Administrator audience. So include the kinds of things they would want to know about: summaries of important changes, how to install, known bugs, anything your software might do that would make someone pull their hair out, etc.
