Create Installation.md

Closes #201

docs/Installation.md

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+# Wasm3 installation
+
+## Windows
+Download `wasm3-win-x64.exe` or `wasm3-win-x86.exe` from the [latest release page](https://github.com/wasm3/wasm3/releases/latest).
+
+## macOS
+Use [Homebrew](https://brew.sh) to build and install automatically:
+```sh
+brew install wasm3
+```
+
+## Cosmopolitan / Actually Portable Executable
+[APE](https://justine.lol/cosmopolitan/index.html) is a polyglot format that runs natively on Linux + Mac + Windows + FreeBSD + OpenBSD + NetBSD + BIOS.  
+Download `wasm3-cosmopolitan.com` from the [latest release page](https://github.com/wasm3/wasm3/releases/latest)
+
+## OpenWRT
+Check instructions [here](https://github.com/wasm3/wasm3-openwrt-packages).