cmd/oci-image-tool/main.go: Add description with supported versions

This shows up in the --help output.

I expect image-tools to support a range of image-spec versions, not
just a single one.  For example, once image-spec cuts 2.0, image-tools
should still be able to process 1.x images for some reasonable
deprecation period.  image-spec is setting us up for that possibility
by using the v1 package for the Go types that are likely to change.
And SemVer means that if we can unpack a 1.3 image, we can also unpack
1.2, 1.1, and 1.0 images.  So I think we want this to be printing:

  * OCI Image Format Specification: 1.0.0-rc5

now.  Once 1.0 is cut we should show:

  * OCI Image Format Specification: 1.0.0

Once 1.1 is cut we should show our position in the 1.x series:

  * OCI Image Format Specification: 1.1.0

And then when 2.0 is cut we should show our positions in both the 1.x
and 2.x series.  If 2.0 happened after 1.1 (without a 1.2, etc.), that
would be:

  * OCI Image Format Specification: 1.1.0, 2.0.0

I see less need to support multiple runtime-spec formats, since
they're only used in 'oci-image-tool create ...'.  But the approach I
used for image-spec ports well to runtime-spec, so I've listed a
version for it too.

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

Author: wking

cmd/oci-image-tool/main.go

@@ -20,6 +20,8 @@ import (
 
 	"github.com/Sirupsen/logrus"
 	"github.com/opencontainers/image-tools/version"
+	image_spec "github.com/opencontainers/image-spec/specs-go"
+	runtime_spec "github.com/opencontainers/runtime-spec/specs-go"
 	"github.com/urfave/cli"
 )
 
@@ -35,6 +37,7 @@ func main() {
 	} else {
 		app.Version = version.Version
 	}
+	app.Description = fmt.Sprintf("Tools for working with OCI images.  Currently supported specifications are:\n\n   * OCI Image Format Specification: %s\n   * OCI Runtime Specification: %s", image_spec.Version, runtime_spec.Version)
 	app.Usage = "OCI (Open Container Initiative) image tools"
 	app.Flags = []cli.Flag{
 		cli.BoolFlag{