update docs with current supported features options

Signed-off-by: Anda Xu <[email protected]>

docs/reference/commandline/dockerd.md

@@ -1423,6 +1423,15 @@ This is a full example of the allowed configuration options on Windows:
     "insecure-registries": []
 }
 ```
+#### Feature options
+The optional field `features` in `daemon.json` grants docker users more granularity to 
+enable/disable specific features. For example, `{"features":{"buildkit": true}}` enables 
+`buildkit` as the default docker image builder.
+
+The list of currently supported feature options:
+- `buildkit`: It enables `buildkit` as default builder when set to `true` or disables it by
+`false`. Note that if this option is not explicitly set in the daemon config file, then it
+is up to the cli to determine which builder to invoke.
 
 #### Configuration reload behavior