Update Emscripten readme about emrun (#3412)

examples/example_emscripten/README.md

@@ -1,19 +1,20 @@
 
-# How to Build
+## How to Build
 
 - You need to install Emscripten from https://emscripten.org/docs/getting_started/downloads.html, and have the environment variables set, as described in https://emscripten.org/docs/getting_started/downloads.html#installation-instructions
-
+- You may also refer to our [Continuous Integration setup](https://github.com/ocornut/imgui/tree/master/.github/workflows) for Emscripten setup.
 - Depending on your configuration, in Windows you may need to run `emsdk/emsdk_env.bat` in your console to access the Emscripten command-line tools.
-
 - Then build using `make` while in the `example_emscripten/` directory.
 
-- 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>
+## How to Run
+
+To run on a local machine:
+- Generally 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/."_
+- Emscripten SDK has a handy `emrun` command: `emrun example_emscripten.html` which will spawn a temporary local webserver. See https://emscripten.org/docs/compiling/Running-html-files-with-emrun.html for details.
+- Otherwise you may use Python builtin webserver: `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:
+## 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`
+- 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`