I’ve always gone for a document that expands fractally - that is to say, the title is clear, the opening paragraph gives an overview that a stakeholder would want, the following paragraphs give top level detail for a quick skim/end users, and then you delve into the detail of each feature/fix/etc.

The first sentence should inform of any breaking changes or major (e.g. security) fixes. For instance:

We are pleased to announce the release of version 2.2.1, which includes several fixes for major issues, includes one change which will require action on your part, 36 bug fixes, 12 enhancements, and the new combine harvester feature.

And then you drill into the top level. Also, categorise, it helps people find stuff.