Add a deprecation document detailing signature store removal

Signed-off-by: Richard Scothern <richard.scothern@docker.com>

cmd/registry/config-dev.yml

@@ -24,7 +24,7 @@ storage:
     cache:
         blobdescriptor: redis
     filesystem:
-        rootdirectory: /var/lib/registry
+        rootdirectory: /var/lib/registry-hello-world-clean
     maintenance:
         uploadpurging:
             enabled: false

docs/deprecated.md

@@ -0,0 +1,24 @@
+<!--[metadata]>
++++
+title = "registry deprecation"
+description = "describes deprecated functionality"
+keywords = ["registry, manifest, images, signatures, repository, distribution, digest"]
++++
+<![end-metadata]-->
+
+# Docker Registry Deprecation
+
+This document details functionality or components which are deprecated within
+the registry.
+
+### v2.5.0
+
+The signature store has been removed from the registry.  Since `v2.4.0` it has
+been possible to configure the registry to generate manifest signatures rather
+than load them from storage.   In this version of the registry this becomes
+the default behavior.  Signatures which are attached to manifests on put are
+not stored in the registry.  This does not alter the functional behavior of
+the registry.
+
+Old signatures blobs can be removed from the registry storage by running the
+garbage-collect subcommand.