Android event documentation - Adding part about mapping event names.

Summary:
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I was quite lost when the documentation told me to go figure the event mapping out myself. It took me quite a while to figure out that i needed to register the names in the `ViewManager`.

This code snippet just makes it way easier to figure out what you need to do to add events.
Closes https://github.com/facebook/react-native/pull/16293

Differential Revision: D6060595

Pulled By: ericnakagawa

fbshipit-source-id: c4755cdb8d99797ff5248ec4eb7e58e2f7ac2588

docs/NativeComponentsAndroid.md

@@ -144,7 +144,24 @@ class MyCustomView extends View {
 }
 ```
 
-The event name `topChange` maps to the `onChange` callback prop in JavaScript (mappings are in `UIManagerModuleConstants.java`). This callback is invoked with the raw event, which we typically process in the wrapper component to make a simpler API:
+To map the `topChange` event name to the `onChange` callback prop in JavaScript, register it by overriding the `getExportedCustomBubblingEventTypeConstants` method in your `ViewManager`:
+
+```java
+public class ReactImageManager extends SimpleViewManager<MyCustomView> {
+    ...
+    public Map getExportedCustomBubblingEventTypeConstants() {
+        return MapBuilder.builder()
+            .put(
+                "topChange",
+                MapBuilder.of(
+                    "phasedRegistrationNames",
+                    MapBuilder.of("bubbled", "onChange")))
+                    .build();
+    }
+}
+```
+
+This callback is invoked with the raw event, which we typically process in the wrapper component to make a simpler API:
 
 ```js
 // MyCustomView.js