Docs: Add note about plugin opt-in for controls

packages/data/README.md

@@ -129,6 +129,8 @@ The `resolvers` option should be passed as an object where each key is the name
 
 ### `controls`
 
+_**Note:** Controls are an opt-in feature, enabled via `use` (the [Plugins API](https://github.com/WordPress/gutenberg/tree/master/packages/data/src/plugins))._
+
 A **control** defines the execution flow behavior associated with a specific action type. This can be particularly useful in implementing asynchronous data flows for your store. By defining your action creator or resolvers as a generator which yields specific controlled action types, the execution will proceed as defined by the control handler.
 
 The `controls` option should be passed as an object where each key is the name of the action type to act upon, the value a function which receives the original action object. It should returns either a promise which is to resolve when evaluation of the action should continue, or a value. The value or resolved promise value is assigned on the return value of the yield assignment. If the control handler returns undefined, the execution is not continued.