specs-go: adding structure of oci-layout file

This file found in an image-layout only had a described content, but not
a referenced structure.

Signed-off-by: Vincent Batts <[email protected]>

Author: vbatts

specs-go/v1/layout.go

@@ -0,0 +1,7 @@
+package v1
+
+// ImageLayout is the structure in the "oci-layout" file, found in the root
+// of an OCI Image-layout directory.
+type ImageLayout struct {
+	Version string `json:"imageLayoutVersion"`
+}