Add note about external-stdlib config in libraries section

Author: ryyppy

pages/docs/manual/latest/libraries.mdx

@@ -14,6 +14,8 @@ ReScript libraries are just like JavaScript libraries: published & hosted on [NP
 
 We recommend you to check in your compiled JavaScript output, for its [various benefits](interop-with-js-build-systems.md#popular-js-build-systems). If not, then at least consider publishing the JavaScript output by un-ignoring them in your [npmignore](https://docs.npmjs.com/cli/v7/using-npm/developers#keeping-files-out-of-your-package). This way, your published ReScript package comes with plain JavaScript files that JS users can consume. If your project's good, JS users might not even realize that they've installed a library written in ReScript!
 
+In case your library is only consumed by JS users, you may want to check out our [external stdlib](./build-external-stdlib) configuration as well.
+
 ### Find Libraries
 
 Search `rescript`-related packages on NPM, or use our [Package Index](/packages).