docs(README): add usage example

README.md

@@ -20,3 +20,72 @@
 ```
 ember install ember-render-helpers
 ```
+
+## Usage
+
+### Example
+
+Clicking the `Toggle` button will toggle the `isVisible` flag. When it switches
+to `true`, `onDidInsert` will be called, because the `{{did-insert}}` helper is
+inserted. When it switches to `false`, `onWillDestroy` will be called, because
+the `{{will-destroy}}` helper is removed.
+
+Clicking the `Random` button will set `randomValue` to a new random value. Every
+time this happens, while `isVisible` is `true`, `onDidUpdate` will be called,
+because one of the parameters passed to the `{{did-update}}` helper was updated.
+
+Clicking the `Random` button _does not_ cause `{{did-insert}}` or
+`{{will-destroy}}` to call `onDidInsert` and `onWillDestroy`, since these
+helpers are not triggered by parameter updates.
+
+```hbs
+{{#if this.isVisible}}
+  {{did-insert   this.onDidInsert   1 2 3 this.randomValue foo="bar" qux="baz"}}
+  {{will-destroy this.onWillDestroy 1 2 3 this.randomValue foo="bar" qux="baz"}}
+  {{did-update   this.onDidUpdate   1 2 3 this.randomValue foo="bar" qux="baz"}}
+{{/if}}
+
+
+<button {{on "click" this.toggleVisibility}}>Toggle</button>
+<button {{on "click" this.rollTheDice}}>Random</button>
+```
+
+```ts
+import { action } from '@ember/object';
+import Component from '@glimmer/component';
+import { tracked } from '@glimmer/tracking';
+
+export default class extends Component {
+  @tracked isVisible = false;
+
+  @tracked randomValue?: number;
+
+  @action
+  toggleVisibility() {
+    this.isVisible = !this.isVisible;
+  }
+
+  @action
+  rollTheDice() {
+    this.randomValue = Math.random();
+  }
+
+  @action
+  onDidInsert(positional: unknown[], named: Record<string, unknown>) {
+    console.log({ positional, named });
+    // => { positional: [1, 2, 3, 0.1337...], named: { foo: 'bar', qux: 'baz' } }
+  }
+
+  @action
+  onWillDestroy(positional: unknown[], named: Record<string, unknown>) {
+    console.log({ positional, named });
+    // => { positional: [1, 2, 3, 0.1337...], named: { foo: 'bar', qux: 'baz' } }
+  }
+
+  @action
+  onDidUpdate(positional: unknown[], named: Record<string, unknown>) {
+    console.log({ positional, named });
+    // => { positional: [1, 2, 3, 0.1337...], named: { foo: 'bar', qux: 'baz' } }
+  }
+}
+```