docs: add section on binding to passive events (angular#42292)

fixes angular#41622

PR Close angular#42292

Author: David Shevitz

aio/content/guide/event-binding.md

@@ -24,6 +24,26 @@ The event binding listens for the button's click events and calls the component'
   <img src='generated/images/guide/template-syntax/syntax-diagram.svg' alt="Syntax diagram">
 </div>
 
+## Binding to passive events
+
+Angular also supports passive event listeners. For example, you can use the following steps to make a scroll event passive.
+
+1. Create a file `zone-flags.ts` under `src` directory.
+2. Add the following line into this file.
+
+```
+(window as any)['__zone_symbol__PASSIVE_EVENTS'] = ['scroll'];
+```
+
+3. In the `src/polyfills.ts` file, before importing zone.js, import the newly created `zone-flags`.
+
+```
+import './zone-flags';
+import 'zone.js';  // Included with Angular CLI.
+```
+
+After those steps, if you add event listeners for the `scroll` event, the listeners will be `passive`.
+
 ## Custom events with `EventEmitter`
 
 [Directives](guide/built-in-directives) typically raise custom events with an Angular [EventEmitter](api/core/EventEmitter) as follows.