README: document the `prebuilt` option

README.md

@@ -55,6 +55,10 @@ Login as `root` with an empty password. Use `build-container $(ls -1
 If you encounter `too many open files` errors, bump limits for virtio
 with `ulimit -n 524288` before starting QEMU.
 
+For development purposes `build-container` can pick up container
+rootfs built outside QEMU when the container's `prebuilt` option is
+set.
+
 #### Gerätekonfigurationen
 
 ```