image/*/interface: Add unstable warnings to Engines

As called for during today's meeting [1].  The wording is from Brandon
[2].

[1]: http://ircbot.wl.linuxfoundation.org/meetings/opencontainers/2016/opencontainers.2016-07-27-21.00.log.html#l-54
[2]: https://github.com/opencontainers/image-spec/pull/159/files/9f8a18eb1cb2a1cbccd6fe137d743dccc2142901#r72525379

Signed-off-by: W. Trevor King <wking@tremily.us>

image/cas/interface.go

@@ -22,6 +22,10 @@ import (
 )
 
 // Engine represents a content-addressable storage engine.
+//
+// This interface is for internal use of oci-image-tool for the time
+// being.  It is subject to change.  This notice will be removed when
+// and if the interface becomes stable.
 type Engine interface {
 
 	// Put adds a new blob to the store.  The action is idempotent; a

image/refs/interface.go

@@ -25,6 +25,10 @@ import (
 type ListNameCallback func(ctx context.Context, name string) (err error)
 
 // Engine represents a name-based reference storage engine.
+//
+// This interface is for internal use of oci-image-tool for the time
+// being.  It is subject to change.  This notice will be removed when
+// and if the interface becomes stable.
 type Engine interface {
 
 	// Put adds a new reference to the store.  The action is idempotent;