Add benchmark chart images to sheetz-benchmarks README

[chitralabs]

Improvement: Visual Benchmark Charts

The sheetz-benchmarks README has great benchmark tables but no visual charts.
A bar chart image showing Sheetz vs Apache POI would be much more shareable on Reddit/Twitter.

Task:

1. Use JMH Visualizer (https://jmh.morethan.io) with results/results.txt
2. Export chart images as PNG
3. Add to results/ directory
4. Reference in README with ![Chart](results/write-chart.png)

Difficulty: Very Easy — no Java coding required
Time estimate: ~30 minutes