docs: including additional assets into libraries (angular#44076)

PR Close angular#44076

Author: nivekcode

aio/content/guide/angular-package-format.md

@@ -107,6 +107,10 @@ Tooling that is aware of these keys may preferentially select a desirable code f
 
 * `"default"` selects flattened ES2020 code for all other consumers.
 
+Libraries may want to expose additional static files which are not captured by the exports of the JavaScript-based entry-points such as Sass mixins or pre-compiled CSS.
+
+For more information, see [Managing assets in a library](/guide/creating-libraries#managing-assets-in-a-library).
+
 ### Legacy resolution keys
 
 In addition to `"exports"`, the top-level `package.json` also defines legacy module resolution keys for resolvers that don't support `"exports"`. For `@angular/core` these are:

aio/content/guide/creating-libraries.md

@@ -152,6 +152,29 @@ npm publish
 In your Angular library, the distributable can include additional assets like theming files, Sass mixins, or documentation (like a changelog).
 For more information [copy assets into your library as part of the build](https://github.com/ng-packagr/ng-packagr/blob/master/docs/copy-assets.md) and [embed assets in component styles](https://github.com/ng-packagr/ng-packagr/blob/master/docs/embed-assets-css.md).
 
+<div class="alert is-important">
+
+When including additional assets like Sass mixins or pre-compiled CSS. You need to add these manually to the conditional ["exports"](guide/angular-package-format/#exports) in the `package.json` of the primary entrypoint.
+
+`ng-packagr` will merge handwritten `"exports"` with the auto-generated ones, allowing for library authors to configure additional export subpaths, or custom conditions.
+
+<code-example language="json">
+"exports": {
+  ".": {
+    "sass": "./_index.scss",
+  },
+  "./theming": {
+    "sass": "./_theming.scss"
+  },
+  "./prebuilt-themes/indigo-pink.css": {
+    "style": "./prebuilt-themes/indigo-pink.css"
+  }
+}
+</code-example>
+
+The above is an extract from the [@angular/material](https://unpkg.com/browse/@angular/material/package.json) distributable.
+</div>
+
 ## Peer dependencies
 Angular libraries should list any `@angular/*` dependencies the library depends on as peer dependencies.
 This ensures that when modules ask for Angular, they all get the exact same module.