Update build-external-stdlib.mdx

Fixes: rescript-lang/rescript#6754

Author: TristanCacqueray

pages/docs/manual/latest/build-external-stdlib.mdx

@@ -24,10 +24,12 @@ To solve that, we now publish our runtime as a standalone package at [`@rescript
 Say you want to publish a JS-only ReScript 9.0 library. Install the packages like this:
 
 ```sh
-npm install [email protected] --save-dev
+npm install [email protected]
 npm install @rescript/[email protected]
 ```
 
+> Note: upcoming std version will contain the Core library, then `rescript` could be installed as a `--dev` dependencies.
+
 Then add this to `rescript.json`:
 
 ```json
@@ -37,6 +39,21 @@ Then add this to `rescript.json`:
 }
 ```
 
+Make sure you are using commonjs because only `.mjs` files are published:
+
+```json
+{
+  // ...
+  "package-specs": [
+    {
+      "module": "esmodule",
+      "in-source": true
+    }
+  ],
+  "suffix": ".mjs",
+}
+```
+
 Now the compiled JS code will import using the path defined by `external-stdlib`. Check the JS output tab:
 
 <CodeTab labels={["ReScript", "JS output"]}>