Update Emscripten readme about local XHR requests (#3412)

examples/example_emscripten/README.md

@@ -7,6 +7,13 @@
 
 - Then build using `make` while in the `example_emscripten/` directory.
 
-- Note that Emscripten 1.39.0 (October 2019) obsoleted the `BINARYEN_TRAP_MODE=clamp` compilation flag which was required with version older than 1.39.0 to avoid rendering artefacts. See [#2877](https://github.com/ocornut/imgui/issues/2877) for details. If you use an older version, uncomment this line in the Makefile:
+- For local testing, you may need a local webserver. Quoting [https://emscripten.org/docs/getting_started](https://emscripten.org/docs/getting_started/Tutorial.html#generating-html):<br>
+_"Unfortunately several browsers (including Chrome, Safari, and Internet Explorer) do not support file:// [XHR](https://emscripten.org/docs/site/glossary.html#term-xhr) requests, and can’t load extra files needed by the HTML (like a .wasm file, or packaged file data as mentioned lower down). For these browsers you’ll need to serve the files using a [local webserver](https://emscripten.org/docs/getting_started/FAQ.html#faq-local-webserver) and then open http://localhost:8000/hello.html."_
+<br>Easy local webserver: _"For example, Python has one built in, `python -m http.server` in Python 3 or `python -m SimpleHTTPServer` in Python 2. After doing that, you can visit http://localhost:8000/."_
+
+Obsolete features:
+
+- Emscripten 2.0 (August 2020) obsoleted the fastcomp back-end, only llvm is supported.
+- Emscripten 1.39.0 (October 2019) obsoleted the `BINARYEN_TRAP_MODE=clamp` compilation flag which was required with version older than 1.39.0 to avoid rendering artefacts. See [#2877](https://github.com/ocornut/imgui/issues/2877) for details. If you use an older version, uncomment this line in the Makefile:
 
 `#EMS += -s BINARYEN_TRAP_MODE=clamp`