docs: Note the lowest supported Emscripten version.

Reference Issue #6304.

docs/README-emscripten.md

@@ -1,6 +1,10 @@
 Emscripten
 ================================================================================
 
+SDL currently requires at least Emscripten 2.0.32 to build. Newer versions
+are likely to work, as well.
+
+
 Build:
 
     $ mkdir build