Document for third-party renderers

packages/react-reconciler/README.md

@@ -211,6 +211,32 @@ You can proxy this to `queueMicrotask` or its equivalent in your environment.
 
 This is a property (not a function) that should be set to `true` if your renderer is the main one on the page. For example, if you're writing a renderer for the Terminal, it makes sense to set it to `true`, but if your renderer is used *on top of* React DOM or some other existing renderer, set it to `false`.
 
+#### `getCurrentEventPriority`
+
+To implement this method, you'll need some constants available on the _returned_ `Renderer` object:
+
+```js
+const HostConfig = {
+  // ...
+  getCurrentEventPriority() {
+    return MyRenderer.DefaultEventPriority;
+  },
+  // ...
+}
+
+const MyRenderer = Reconciler(HostConfig);
+```
+
+The constant you return depends on which event, if any, is being handled right now. (In the browser, you can check this using `window.event && window.event.type`).
+
+* **Discrete events:** If the active event is _directly caused by the user_ (such as mouse and keyboard events) and _each event in a sequence is intentional_ (e.g. `click`), return `MyRenderer.DiscreteEventPriority`. This tells React that they should interrupt any background work and cannot be batched across time.
+
+* **Continuous events:** If the active event is _directly caused by the user_ but _the user can't distinguish between individual events in a sequence_ (e.g. `mouseover`), return `MyRenderer.ContinuousEventPriority`. This tells React they should interrupt any background work but can be batched across time.
+
+* **Other events / No active event:** In all other cases, return `MyRenderer.DefaultEventPriority`. This tells React that this event is considered background work, and interactive events will be prioritized over it.
+
+You can consult the `getCurrentEventPriority()` implementation in `ReactDOMHostConfig.js` for a reference implementation.
+
 ### Mutation Methods
 
 If you're using React in mutation mode (you probably do), you'll need to implement a few more methods.