doc: mention external PKGBUILDs for ArchLinux Emscripten builds.
doc: mention external PKGBUILDs for ArchLinux Emscripten builds.

mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples,mosra/magnum-examples