Add supportsMicrotask to react-reconciler README

Author: acdlite

packages/react-reconciler/README.md

@@ -203,9 +203,12 @@ You can proxy this to `clearTimeout` or its equivalent in your environment.
 
 This is a property (not a function) that should be set to something that can never be a valid timeout ID. For example, you can set it to `-1`.
 
+#### `supportsMicrotask`
+
+Set this to true to indicate that your renderer supports `scheduleMicrotask`. We use microtasks as part of our discrete event implementation in React DOM. If you're not sure if your renderer should support this, you probably should. The option to not implement `scheduleMicrotask` exists so that platforms with more control over user events, like React Native, can choose to use a different mechanism.
 #### `scheduleMicrotask(fn)`
 
-You can proxy this to `queueMicrotask` or its equivalent in your environment.
+Optional. You can proxy this to `queueMicrotask` or its equivalent in your environment.
 
 #### `isPrimaryRenderer`