write introduction for optional features section

Author: locks

guides/v3.3.0/optional-features/introduction.md

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+Because major releases of Ember are not supposed to make breaking changes without prior deprecation, the project has been extremely conservative about changing behaviors that don't have a clear deprecation path. As a result, we've had several quirks of the framework linger into the 3.x series.
+
+To give the project a path forward when a breaking change is mandatory, we've released the [`@ember/optional-features`](https://github.com/emberjs/ember-optional-features) addon.
+
+This addon does nothing by default, but provides a command-line interface to enable and disable breaking changes in Ember.
+
+## Installation
+
+To get started with optional features, you must install the addon:
+
+```bash
+ember install @ember/optional-features
+```
+
+This will make three new commands available to Ember CLI within your project, `feature:list`, `feature:enable`, and `feature:disable`.
+
+## Listing features
+
+The optional features available to your project will depend on the Ember version your project is using.
+
+To see which optional features are available, you can run the following command:
+
+```bash
+$ ember feature:list
+Usage:
+
+  To list all available features, run ember feature:list.
+  To enable a feature, run ember feature:enable some-feature.
+  To disable a feature, run ember feature:disable some-feature.
+
+Available features:
+
+  application-template-wrapper (Default: true)
+    Wrap the top-level application template (application.hbs) with a `<div class="ember-view">` element.
+    More information: https://github.com/emberjs/rfcs/pull/280
+
+  jquery-integration (Default: true)
+    Adds jQuery to the Ember application.
+    More information: https://github.com/emberjs/rfcs/pull/294
+
+```
+
+## Enabling or disabling a feature
+
+Once you see a feature that you would like to toggle for your project you can run one of two commands, `ember feature:enable <feature>` and `ember feature:disable <feature>`.
+
+Let us disable `template-only-glimmer-components` to see what happens:
+
+```bash
+$ ember feature:disable template-only-glimmer-components
+Disabled template-only-glimmer-components. Be sure to commit config/optional-features.json to source control!
+```
+
+As we can see from the warning, `@ember/optional-features` has created a file in `config/optional-features.json` to store the configuration for your project.
+We commit it to our repository and we are off to the races!