[docs] Clarify that merely changing extension does not load .md f…

packages/next-mdx/readme.md

@@ -51,7 +51,7 @@ module.exports = withMDX({
 })
 ```
 
-Optionally you can match other file extensions for MDX compilation, by default only `.mdx` is supported
+Optionally you can match other file extensions for MDX compilation, as by default only `.mdx` is supported:
 
 ```js
 // next.config.js
@@ -61,6 +61,21 @@ const withMDX = require('@next/mdx')({
 module.exports = withMDX()
 ```
 
+The configuration above will load both `.md` and `.mdx` files. However, please note that even with this configuration in place, MDX will load `.md` files as Markdown (not as MDX) because by default it will auto-detect the type.
+If you want to load `.md` files as MDX content, you should in addition configure [the `format` option](https://mdxjs.com/packages/mdx/#optionsformat) and set it to `mdx`:
+
+```js
+// next.config.js
+const withMDX = require('@next/mdx')({
+  extension: /\.(md|mdx)$/,
+  options: {
+    format: 'mdx'
+  }
+})
+module.exports = withMDX()
+```
+
+
 ## Top level .mdx pages
 
 Define the `pageExtensions` option to have Next.js handle `.md` and `.mdx` files in the `pages` directory as pages: