Add a note about using asset pipeline assets (#140)

* Add a note about using asset pipeline assets

* Rewrite note about using assets from the pipeline

Clarify how it works so readers don't have to open the Sprockets PR.
Also remove any mention of how it used to work, this should be an
evergreen README comment.

README.md

@@ -93,6 +93,27 @@ A fix for this has been proposed upstream at https://github.com/tailwindlabs/tai
 apk add build-base gcompat
 ```
 
+### Using asset-pipeline assets
+
+In Rails, you want to use [assets from the asset pipeline to get fingerprinting](https://guides.rubyonrails.org/asset_pipeline.html#what-is-fingerprinting-and-why-should-i-care-questionmark). However, Tailwind isn't aware of those assets. To use assets from the pipeline, use `url(image.svg)`. [Since Sprockets v3.3.0](https://github.com/rails/sprockets-rails/pull/476) `url(image.svg)` will then automatically be rewritten to `/path/to/assets/image-7801e7538c6f1cc57aa75a5876ab0cac.svg`. So the output CSS will have the correct path to those assets.
+
+```js
+module.exports = {
+    theme: {
+        extend: {
+            backgroundImage: {
+                'image': "url('image.svg')"
+            }
+        }
+    }
+}
+```
+
+The inline version also works:
+
+```html
+<section class="bg-[url('image.svg')]">Has the image as it's background</section>
+```
 
 ## License