docs: add CPU_temp to the projects index

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

README.md

@@ -14,3 +14,9 @@ A collection of Arduino / ESP microcontroller projects.
   panel, and posts events to an HTTPS webhook. Settings live in LittleFS.
   See its [README](pump_controller_8_2_OLED_DONE/README.md) and
   [MANUAL](pump_controller_8_2_OLED_DONE/MANUAL.md).
+
+- **[CPU_temp](CPU_temp/)** — desktop CPU temperature monitor on an ESP32-C3 with
+  a built-in 0.42-inch OLED: a Windows agent reads the sensors and feeds the
+  board over USB serial, which shows the temperature, CPU load and a one-minute
+  history graph, and blinks an overheat alarm. See its
+  [README](CPU_temp/README.md).