The first thing anyone reads.
WHAT IT MUST ANSWER
What is this? Who is it for? How do I set it up? How do I run it? How do I run the tests? How do I deploy it? Where do I go for more?
WHAT TO PUT FIRST
What it is, in one or two sentences.
WHY
Readers decide within seconds whether it is what they need.
WHAT TO INCLUDE IN SETUP INSTRUCTIONS
Every prerequisite, with versions Every step, in order What success looks like
WHAT TO TEST
The instructions, by following them on a clean machine.
WHAT THAT REVEALS
Every step that was never written down.
WHAT ELSE TO INCLUDE
How to contribute Who maintains it Where to report problems
WHAT TO KEEP OUT
Extensive documentation better placed elsewhere Anything that will become out of date immediately
WHAT TO UPDATE
It, whenever setup or running changes.
WHY IT MATTERS MOST OF ALL DOCUMENTATION
It determines whether anyone can use the project at all.