Sample config file changes

Rename config.yml to dev-config.yml

Add example-config.yml, a simple configuration file for the official
This was originally made for the the distribution-library-image repo,
but is being moved here to make sure it stays in sync.

Update Dockerfile and docs for the rename.

Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>

cmd/registry/example-config.yml

@@ -0,0 +1,14 @@
+version: 0.1
+log:
+  fields:
+    service: registry
+storage:
+    cache:
+        layerinfo: inmemory
+    filesystem:
+        rootdirectory: /var/lib/registry
+    maintenance:
+        uploadpurging:
+            enabled: false
+http:
+    addr: :5000