Add working docs on WMR

Related-to GH-62.
Closes GH-69.

Reviewed-by: Titus Wormer <tituswormer@gmail.com>

readme.md

@@ -1424,13 +1424,22 @@ your `vite.config.js`.
 
 #### WMR
 
-~~[WMR](https://github.com/preactjs/wmr) supports [Rollup][] plugins directly by
-[pushing them to `config.plugins`](https://github.com/preactjs/wmr/tree/main/packages/wmr#configuration-and-plugins)
-in `wmr.config.js`.~~
+[WMR](https://github.com/preactjs/wmr) supports [Rollup][] plugins directly by
+[adding them to `plugins`](https://wmr.dev/docs/plugins/)
+in `wmr.config.mjs`.
 
-WMR *says* it supports Rollup plugin but apparently it fails on `.mdx` files
-before the Rollup plugin.
-See [GH-62](https://github.com/wooorm/xdm/issues/62).
+```js
+import {defineConfig} from 'wmr'
+import xdm from 'xdm/rollup.js'
+
+export default defineConfig({
+  plugins: [
+    xdm({/* Options… */})
+  ]
+})
+```
+
+See [§ Preact](https://github.com/wooorm/xdm#preact) if you want to use Preact.
 
 ### Compilers