doc: qemu-riscv: describe attaching a disk image

How to add an emulated disk to QEMU may not be known to the user.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
diff --git a/doc/board/emulation/qemu-riscv.rst b/doc/board/emulation/qemu-riscv.rst
index b68db95..3acd406 100644
--- a/doc/board/emulation/qemu-riscv.rst
+++ b/doc/board/emulation/qemu-riscv.rst
@@ -105,3 +105,11 @@
 
     qemu-system-riscv64 -nographic -machine virt -bios spl/u-boot-spl \
     -device loader,file=u-boot.itb,addr=0x80200000
+
+An attached disk can be emulated by adding::
+
+    -device ich9-ahci,id=ahci \
+    -drive if=none,file=riscv64.img,format=raw,id=mydisk \
+    -device ide-hd,drive=mydisk,bus=ahci.0
+
+You will have to run 'scsi scan' to use it.