update README about new push behavior

With 4f3cb87 (cnabio#76) and 5a97c84 (cnabio#78) `cnab-to-oci` is now able
to push images from local docker daemon image store.

Signed-off-by: Yves Brissaud <yves.brissaud@docker.com>

README.md

@@ -224,8 +224,9 @@ Copying descriptor sha256:a59a4e74d9cc89e4e75dfb2cc7ea5c108e4236ba6231b53081a9e2
 Pushed successfully, with digest "sha256:6cabd752cb01d2efb9485225baf7fc26f4322c1f45f537f76c5eeb67ba8d83e0"
 ```
 
-**Note:** `cnab-to-oci` does not push images from your docker daemon image store.
-Make sure all your invocation images are already present on a registry before pushing your bundle.
+**Note:** if your images -invocation images as well as service images- are not already
+pushed on a registry, `cnab-to-oci` will try to resolve them locally and push them
+from your docker daemon image store.
 
 **Note:** The `MANIFEST_INVALID` error in the above case is because the Docker Hub
 does not currently support the OCI image index type.