Update README and add additional docs for rule

Author: hxqlin

README.md

@@ -138,6 +138,7 @@ Rules are grouped by category to help you understand their purpose. Each rule ha
 | :car: | [no-actions-hash](./docs/rules/no-actions-hash.md) | disallow the actions hash in components, controllers, and routes |
 | :car: | [no-classic-classes](./docs/rules/no-classic-classes.md) | disallow "classic" classes in favor of native JS classes |
 | :white_check_mark::wrench: | [no-ember-super-in-es-classes](./docs/rules/no-ember-super-in-es-classes.md) | disallow use of `this._super` in ES class methods |
+| :car: | [no-empty-glimmer-component-classes](./docs/rules/no-empty-glimmer-component-classes.md) | disallow empty backing classes for Glimmer components |
 
 ### jQuery
 

docs/rules/no-empty-glimmer-component-classes.md

@@ -4,7 +4,9 @@ This rule will catch and prevent the use of empty backing classes for Glimmer co
 
 ## Rule Details
 
-This rule aims to disallow the use of empty backing classes for Glimmer components. Template-only Glimmer components where there is no backing class are much faster and lighter-weight than Glimmer components with backing classes, which are much lighter-weight than Ember components. Therefore, you should only have a backing class for a Glimmer component when absolutely necessary. Otherwise, remove the backing class entirely until it is actually needed.
+This rule aims to disallow the use of empty backing classes for Glimmer components when possible. Template-only Glimmer components where there is no backing class are much faster and lighter-weight than Glimmer components with backing classes, which are much lighter-weight than Ember components. Therefore, you should only have a backing class for a Glimmer component when absolutely necessary.
+
+In addons, the empty class should be replaced with a `templateOnly` export. This is because addons can't assume `template-only-glimmer-components` is enabled. In apps, remove the backing class entirely until it is actually needed.
 
 ## Examples
 
@@ -28,6 +30,12 @@ class MyComponent extends Component {
 }
 ```
 
+```js
+import templateOnly from '@ember/component/template-only';
+
+export default templateOnly();
+```
+
 ## References
 
 * [Glimmer Components - Octane Upgrade Guide](https://guides.emberjs.com/release/upgrading/current-edition/glimmer-components/)