Rewrite README to describe the 5-level temperature-driven PWM fan
control (replacing the old relay on/off description), and add a
secrets.h.example template so new clones have a reference.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Fan speed is now auto-selected from a temperature table (levels 1-5 at
28/30/32/34/36C mapping to PWM duties 230/236/242/249/255) with 1C
hysteresis. State and level are published to MQTT for Home Assistant.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
