If you are selling a tool, your README is your sales page. Here is the template I use:
[Product Name]
One sentence: what it does and for whom.
The Problem
2-3 sentences about the pain point.
The Solution
What your tool does. Be specific. Include numbers.
Sample Output
Show what the buyer gets. Screenshots, terminal output, generated results.
Quick Start
3-5 steps from purchase to running. If it takes more than 5 minutes, simplify.
What is Included
File list with descriptions.
Pricing
Price + what that gets them. Compare to alternatives.
FAQ
Top 3 questions buyers ask.
Key principle: show, don't tell. Sample output is worth more than 1000 words of description.