layer: specify that volumes should not be part of changesets

In order to match how Docker handles volumes, we should make it clear
that volumes really should be treated as "external" to the root
filesystem (the will often be duped over).

Signed-off-by: Aleksa Sarai <asarai@suse.de>

layer.md

@@ -188,6 +188,8 @@ Deleted:    /etc/my-app-config
 This reflects the removal of `/etc/my-app-config` and creation of a file and directory at `/etc/my-app.d/default.cfg`.
 `/bin/my-app-tools` has also been replaced with an updated version.
 
+An implementation SHOULD NOT allow any changes inside a [volume specified in `Config.Volumes`](config.md) to be included in a Layer Changeset.
+
 ### Representing Changes
 
 A [tar archive][tar-archive] is then created which contains *only* this changeset: