config.md: clarify the root filesystem path

Signed-off-by: Ma Shimiao <mashimiao.fnst@cn.fujitsu.com>

config.md

@@ -25,6 +25,8 @@ For example, if an implementation is compliant with version 1.0.1 of the spec, i
 **`root`** (object, required) configures the container's root filesystem.
 
 * **`path`** (string, required) Specifies the path to the root filesystem for the container.
+  The path can be an absolute path (starting with /) or a relative path (not starting with /), which is relative to the bundle. e.g.
+  If the absolute path of root filesystem is /to/bundle/rootfs, we can set it or rootfs to`path.
   A directory MUST exist at the path declared by the field.
 * **`readonly`** (bool, optional) If true then the root filesystem MUST be read-only inside the container, defaults to false.