mention shim in readme

packages/util/README.md

@@ -54,6 +54,45 @@ The first argument is allowed to be:
 
 In the Javascript version, you must pass a second argument that is any object with an owner (a `Component` instance works great).
 
+## addonV1Shim
+
+This makes a V2-formatted addon act like a v1 addon in non-embroider builds. It lets addons ship as v2 even though their users aren't using embroider yet.
+
+To use it:
+
+1. Be prepared to do a semver major release of your addon, because the shim demands that consumers of your package must have a new-enough version of ember-auto-import. ember-auto-import is how we get v2 addons into non-embroider builds. You will want to document the minimum supported ember-auto-import version needed by the version of the shim you're using.
+2. Create an `addon-main.js` file that requires and invokes the shim:
+
+   ```js
+   const { addonV1Shim } = require('@embroider/util/shim');
+   module.exports = addonV1Shim(__dirname);
+   ```
+
+3. Update your `package.json` to point at addon-main.js:
+
+   ```json
+   {
+     "ember-addon": {
+       "version": 2,
+       "type": "addon",
+       "main": "addon-main.js"
+     }
+   }
+   ```
+
+### Shim Options
+
+**disabled**: this optional argument lets you control whether your shimmed addon will emit `'app-js'` and `'public-assets'` into non-embroider builds. This _only_ works in non-embroider builds, because under embroider apps just won't import the parts of your addon they don't want. Example:
+
+```js
+module.exports = addonV1Shim(__dirname, {
+  disabled(options) {
+    let welcomeConfig = options['ember-welcome-page'] || {};
+    return process.env.EMBER_ENV === 'production' && !welcomeConfig.enabled;
+  },
+});
+```
+
 ## Contributing
 
 See the [Contributing](CONTRIBUTING.md) guide for details.