docs: add recmaPlugins to README

packages/integrations/mdx/README.md

@@ -505,6 +505,12 @@ export default {
 }
 ```
 
+### recmaPlugins
+
+These are plugins that modify the output [estree](https://github.com/estree/estree) directly. This is useful for modifying or injecting JavaScript variables in your MDX files.
+
+We suggest [using AST Explorer](https://astexplorer.net/) to play with estree outputs, and trying [`estree-util-visit`](https://unifiedjs.com/explore/package/estree-util-visit/) for searching across JavaScript nodes.
+
 ## Examples
 
 - The [Astro MDX example](https://github.com/withastro/astro/tree/latest/examples/with-mdx) shows how to use MDX files in your Astro project.