Add Allow md As An Extension With gatsby-mdx as a react til

react/allow-md-as-an-extension-with-gatsby-mdx.md

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+# Allow md As An Extension With gatsby-mdx
+
+The [gatsby-mdx](https://github.com/ChristopherBiscardi/gatsby-mdx) plugin
+allows you to create pages in a Gatsby project using `.mdx` files. If you
+prefer the `.md` extension on your markdown files, then you can adjust the
+plugin options to allow that.
+
+```json
+// gatsby-config.js
+plugins: [
+  {
+    resolve: `gatsby-mdx`,
+    options: {
+      extensions: [".mdx", ".md"]
+    }
+  }
+]
+```
+
+This tells `gatsby-mdx` to recognize both `.mdx` and `.md` extensions when
+processing files.