docs/zh_CN: Add zh_CN/admin-guide/README.rst

Add translation zh_CN/admin-guide/README.rst, and link it to
zh_CN/admin-guide/index.rst while clean its todo entry.
Signed-off-by: Wu XiangCheng <bobwxc@email.cn>
Reviewed-by: Alex Shi <alex.shi@linux.alibaba.com>
Link: https://lore.kernel.org/r/20210304080131.GA16539@mipcSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/translations/zh_CN/admin-guide/index.rst

@@ -13,9 +13,13 @@ Linux 内核用户和管理员指南
 
 这个初始部分包含总体信息，包括描述内核的README， 关于内核参数的文档等。
 
-Todolist:
+.. toctree::
+   :maxdepth: 1
 
    README
+
+Todolist:
+
    kernel-parameters
    devices
    sysctl/index