doc(doc/make/emscripten): document Emscripten build

doc/make/emscripten.md

@@ -0,0 +1,13 @@
+Compiling lean.js via Emscripten
+--------------------------------
+
+First install Emscripten via your distribution's package manager or [download and install it](https://kripken.github.io/emscripten-site/docs/getting_started/downloads.html) yourself. Then build asm.js and WebAssembly binaries using
+
+```bash
+mkdir -p build/emscripten
+cd build/emscripten
+emconfigure cmake ../../src -DCMAKE_BUILD_TYPE=Emscripten
+make lean_js_js lean_js_wasm
+```
+
+This will produce files `lean_js_js.js`, `lean_js_wasm.js`, and `lean_js_wasm.wasm` in `shell/`, which you can e.g. copy to the `dist/` directory in [lean-web-editor](https://github.com/leanprover/lean-web-editor) instead of calling `./fetch_lean_js.sh` there.

doc/make/index.md

@@ -12,6 +12,7 @@ Platform-Specific Setup
 - [Windows (msys2)](msys2.md)
 - [Windows (Visual Studio)](msvc.md)
 - [macOS](osx-10.9.md)
+- [Emscripten: lean.js](emscripten.md)
 
 Generic Build Instructions
 --------------------------