Add note why UNSTABLE is needed for closure compiler

Author: Swordlash

content/case-study-foreign-integration-js-browser/index.md

@@ -74,6 +74,7 @@ Output size: 396K
 
 Notice we have to call `google-closure-compiler` with two input files: first is the `all.js` package generated by the compiler after bundling our library with all dependencies,
 GHC's RTS and emscripten RTS files; second, is the `all.externs.js` file that declares external variables for the minifier. Its purpose is twofold - It informs the compiler that those variables are declared elsewhere, as not to fail with an "undeclared variable" error, and it prevents the mangling of those identifiers during minification.
+Note we had to add `--language_in UNSTABLE` when compiling for recent `emscripten` due to [this issue](https://github.com/emscripten-core/emscripten/issues/23179).
 
 Nice! We reduced size of the js file from 1.8M to merely 396K. This is even smaller after compression:
 ```sh