Add note on Emscripten version

Author: hoodmane

getting-started/setup-building.rst

@@ -487,6 +487,10 @@ The simplest way to install the Emscripten compiler is:
     ./emsdk/emsdk activate 4.0.5
     source ./emsdk/emsdk_env.sh
 
+Updating the Emscripten compiler version often causes breakages. For the best
+compatibility, use the Emscripten version suggested in the cpython repository in
+`Tools/wasm/README.md`.
+
 Building for Emscripten requires doing a cross-build where you have a *build*
 Python to help produce an Emscripten build of CPython. This means you build
 CPython twice: once to have a version of Python for the build system to use and