manifest: Explicitly unspecified attributes for the initial layer directory

Point out that the layer's initial empty directory has unspecified
attributes.  Layer authors interested in the ownership and other
attributes of the unpacked root directory should explicitly set an
entry for it.

This commit originally focused on requiring the initial directory to
be empty before the layers are unpacked into it.  But most of that was
picked up in fed7241 (manifest: clarify the order of layers,
2016-09-29, opencontainers#349).  I'd recommended "match" instead of "be" to allow
folks to apply via a union filesystem or whatever. As long as the
finished product is right, compliance testers and users should be
satisfied.

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

manifest.md

@@ -161,6 +161,7 @@ Unlike the [Manifest List](#manifest-list), which contains information about a s
     The array MUST have the base image at index 0.
     Subsequent layers MUST then follow in stack order (i.e. from `layers[0]` to `layers[len(layers)]`).
     The final filesystem layout MUST match the result of [applying](layer.md#applying) the layers to an empty directory.
+    The [ownership, mode, and other attributes](layer.md#file-attributes) of the initial empty directory are unspecified.
 
     Beyond the [descriptor requirements](descriptor.md#properties), the value has the following additional restrictions: