Add Upgrading Your Manifest For Sprocket's 4 as a rails til

rails/upgrading-your-manifest-for-sprockets-4.md

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+# Upgrading Your Manifest For Sprocket's 4
+
+If you're upgrading [Rails](https://github.com/rails/rails) and it involves
+bumping the [Sprockets](https://github.com/rails/sprockets) dependency from 3.x
+to 4.x, you may need to update your `manifest.js`.
+
+> Since the default logic for determining top-level targets changed, you might
+> find some files that were currently compiled by sprockets for delivery to
+> browser no longer are. You will have to edit the `manifest.js` to specify
+> those files.
+
+> The `manifest.js` file is meant to specify what files to use as a top-level
+> target using sprockets methods `link`, `link_directory`, and `link_tree`.
+
+You can specify what top-level assets like so:
+
+```javascript
+# app/assets/config/manifest.js
+
+//= link_tree ../images
+//= link_directory ../javascripts .js
+//= link_directory ../stylesheets .css
+//= link some_file.xml
+//= link some/nested/style.css
+```
+
+Read more about the [upgrade process
+here](https://github.com/rails/sprockets/blob/master/UPGRADING.md#manifestjs)
+as well as in [this blog
+post](https://eileencodes.com/posts/the-sprockets-4-manifest/).