copy optional features section to 3.4

Author: locks

guides/v3.4.0/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!

guides/v3.4.0/optional-features/removing-jquery.md

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+Ember relied on jQuery in the past, and it still ships with it by default.
+However, in Ember 3.3 it is an optional dependency that you can opt out from.
+
+To do so, install the [`@ember/optional-features`](https://github.com/emberjs/ember-optional-features) addon
+and disable the `jquery-integration` flag:
+
+```shell
+ember install @ember/optional-features
+ember feature:disable jquery-integration
+```
+
+This will remove jQuery from your `vendor.js` bundle and disable any use of jQuery in Ember itself.
+Now your app will be about 30KB lighter!
+
+## Caveats
+
+Without jQuery, any code that still relies on it will break, especially any usage of
+
+* [`this.$()`](https://www.emberjs.com/api/ember/release/classes/Component/methods/$?anchor=%24) in components
+* `jQuery` or `$` directly as a global, through `Ember.$()` or by importing it (`import jQuery from jquery;`) 
+* global acceptance test helpers like `find()` or `click()`
+* `this.$()` in component tests
+
+Note that this also applies to all addons that your app uses, so make sure they support being used without jQuery.