docs: document enableTypeScriptTransforms

README.md

@@ -19,6 +19,7 @@ allow you to use latest Javascript in your Ember CLI project.
     + [Enabling Source Maps](#enabling-source-maps)
     + [Modules](#modules)
     + [Disabling Debug Tooling Support](#disabling-debug-tooling-support)
+    + [Enabling TypeScript Transforms](#enabling-typescript-transforms)
   * [Addon usage](#addon-usage)
     + [Adding Custom Plugins](#adding-custom-plugins)
     + [Additional Trees](#additional-trees)
@@ -121,6 +122,7 @@ interface EmberCLIBabelConfig {
     disablePresetEnv?: boolean;
     disableEmberModulesAPIPolyfill?: boolean;
     disableDecoratorTransforms?: boolean;
+    enableTypeScriptTransforms?: boolean;
     extensions?: string[];
   };
 }
@@ -259,6 +261,39 @@ module.exports = function(defaults) {
 }
 ```
 
+#### Enabling TypeScript Transforms
+
+The transform plugins required for Babel to transpile TypeScript (including
+transforms required for all valid TypeScript features such as optional
+chaining and nullish coalescing) will automatically be enabled when
+`ember-cli-typescript` >= 4.0 is installed.
+
+You can enable the TypeScript Babel transforms manually *without*
+`ember-cli-typescript` by setting the `enableTypeScriptTransforms`to `true`.
+
+NOTE: Setting this option to `true` is not compatible with
+`ember-cli-typescript` < 4.0 because of conflicting Babel plugin ordering
+constraints and is unnecessary because `ember-cli-typescript` < 4.0 adds the
+TypeScript Babel transforms itself.
+
+NOTE: Setting this option does *not* enable type-checking. For integrated
+type-checking, you will need [`ember-cli-typescript`](https://ember-cli-typescript.com).
+
+In an app, manually enabling the TypeScript transforms would look like:
+
+```js
+// ember-cli-build.js
+module.exports = function(defaults) {
+  let app = new EmberApp(defaults, {
+    'ember-cli-babel': {
+      enableTypeScriptTransforms: true
+    }
+  });
+
+  return app.toTree();
+}
+```
+
 ### Addon usage
 
 #### Adding Custom Plugins