Document current Emscripten version

Beuc · Beuc · commit cc6f1c6023e3 · 2020-09-08T06:59:31.000Z
diff --git a/README.md b/README.md
@@ -13,6 +13,9 @@ See for instance [RenPyWeb](https://github.com/renpy/renpyweb).
 
 Web demo: <https://www.beuc.net/python-emscripten/demo/>
 
+Emscripten evolves regularly with breaking changes.  
+Last tested with Emscripten 1.39.18.
+
 Mirrors:
 
 - <https://gitlab.com/python-emscripten/python>
