Moved registry docs to registry subdirectory

docs/migration.md

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+<!--[metadata]>
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+draft = true
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+<![end-metadata]-->
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+# Migrating a 1.0 registry to 2.0
+
+TODO: This needs to be revised in light of Olivier's work
+
+A few thoughts here:
+
+There was no "1.0". There was an implementation of the Registry API V1 but only a version 0.9 of the service was released.
+The image formats are not compatible in any way. One must convert v1 images to v2 images using a docker client or other tool.
+One can migrate images from one version to the other by pulling images from the old registry and pushing them to the v2 registry.
+
+-----
+
+The Docker Registry 2.0 is backward compatible with images created by the earlier specification. If you are migrating a private registry to version 2.0, you should use the following process:
+
+1. Configure and test a 2.0 registry image in a sandbox environment.
+
+2. Back up up your production image storage.
+
+	Your production image storage should reside on a volume or storage backend.
+	Make sure you have a backup of its contents.
+
+3. Stop your existing registry service.
+
+4. Restart your registry with your tested 2.0 image.