Add doc for wasm.

Author: jianjunz

talk/owt/docs/wasm/wasm.md

@@ -0,0 +1,16 @@
+# Open WebRTC Toolkit Client SDK WebAssembly Build
+
+## Overview
+Open WebRTC Toolkit (OWT) provides a WebAssembly binary to be used with OWT JavaScript SDK for WebTransport based streaming. The only feature it offers now is a RTP depacktizer for video.
+
+This is an experimental feature.
+
+## System requirements
+WebAssembly build is only tested on Ubuntu 20.04 with Emscripten 3.1.0. It may work on other OSes or other versions of Emscripten. Please install [WebRTC stack build dependencies](https://webrtc.googlesource.com/src/+/refs/heads/master/docs/native-code/development/prerequisite-sw/index.md) and [Emscripten](https://emscripten.org/docs/getting_started/downloads.html) before building WebAssembly binaries.
+
+## Build
+To build WebAssembly binary, run `ninja -C <output path> owt_wasm` in `src` directory.
+
+## Known issues
+- Some paths are hard coded. They will be replaced in the future.
+- Only H.264 is supported.