document the multi-sink logging model

docs/README.md

@@ -73,6 +73,10 @@ Cutting across all of these:
 - **[testing.md](testing.md) — testing.** How the kernel is tested by booting it in
   QEMU and asserting on its serial output — reproducibly, and structured so the
   same tests run across architectures.
+- **[logging.md](logging.md) — logging.** The multi-sink diagnostic log (serial,
+  0xE9 debugcon, file later) kept separate from the framebuffer display, plus the
+  robustness path: optional framebuffer, POST-code checkpoints, and a persistent
+  panic breadcrumb so the kernel survives — and can be diagnosed — with no output.
 
 ## How the pieces relate