Tweak README

Author: gaearon

packages/react-reconciler/README.md

@@ -216,10 +216,14 @@ This is a property (not a function) that should be set to `true` if your rendere
 To implement this method, you'll need some constants available on the _returned_ `Renderer` object:
 
 ```js
+// This object contains the constants:
 const MyRenderer = Reconciler(HostConfig);
+// For example, MyRenderer.DefaultEventPriority
 ```
 
-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`):
+The constant you return depends on which event, if any, is being handled right now. In the browser, you can check the current event using `window.event && window.event.type`.
+
+Return one of these values:
 
 * **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.