Add details about object-fit plugin to readme

Author: adamwathan

README.md

@@ -22,6 +22,21 @@ yarn run build
 
 Load up `./index.html` in your browser and you're off to the races!
 
+## Object-Fit Utilities
+
+In `plugins/object-fit/index.js` you'll find an example of a plugin that adds a set of simple, non-configurable utility classes for the `object-fit` property.
+
+The only option it exposes are the variants you'd like to generate (`responsive`, `hover`, `focus`, etc.), which you pass to the plugin as a simple array:
+
+```js
+plugins: {
+    // ...
+    require('./plugins/object-fit')(['responsive']),
+}
+```
+
+This is just about the simplest type of plugin you could make.
+
 ## Simple Buttons
 
 In `plugins/simple-buttons/index.js` you'll find an example of a plugin that adds new component classes.