docs: add ThinkRail to apps built with Electrobun

Summary

Adds ThinkRail to the "Apps Built with Electrobun" list in the README.

ThinkRail is an agentic IDE built around the Pi coding agent. It learns your project through living specs and reusable skills.

Changes

• README.md: adds one entry between Tensamin Client and tokenpass-desktop, keeping the list alphabetical.

Testing

This PR only changes the README, so no code checks apply. I checked the rendered diff by hand.