补充读写权限控制.

Signed-off-by: rick.chan <cy@sina.com>

Ecology/LubanCat2/RK3568_USB3_OTG_说明.md

@@ -116,6 +116,8 @@ usb_acm_en
 .usb_config 文件如下：
 
 ```bash
+# For VBUS_ALWAYS_ON usb otg is not support ums
+# Since the block to ums is always occupated by USB due to no disconneted state
 usb_ums_en
 # 配置挂载的镜像文件地址
 ums_block=/ums_shared.img
@@ -125,6 +127,8 @@ ums_block_size=8
 ums_block_type=fat
 # 设置断开UMS后自动挂载到/mnt/ums目录
 # ums_block_auto_mount=on
+# 读写权限控制
+# UMS_RO=0
 ```
 
 ***注：实测开启 ums_block_auto_mount 后主机无法挂载虚拟存储设备，似乎跟硬件电路有关。***