message: add instruction of compiling wasm_example filter
  Including
  Set up enscripten environment
  With a valid enscripten, build out new .wasm file
diff: @@ -63,3 +63,24 @@ Curl output should include our headers:
  * Connection #0 to host localhost left intact
  }
  ```
  +
  +# Build WASM Binary
  +## prerequisite
  +1. [Download and install `emscripten`](https://emscripten.org/docs/getting_started/downloads.html#installation-instructions)
  +2. Activate emsdk archived version 1.38.25 (Note: by 02/11/2019 the latest build is broken for envoy-wasm)
  +
  +```
  +# in emsdk directory
  +./emsdk install sdk-1.38.25-64bit
  +./emsdk activate sdk-1.38.25-64bit
  +source emsdk_env.sh
  +# verify
  +em++ -v 2>&1 |grep Emscripten ## should output 1.38.25
  +```
  +3. Build wasm filter in envoy directory
  +
  +```shell
  +envoy/examples/wasm$ source <path/to/emsdk>/emsdk_env.sh # Only needed if in another shell session
  +envoy/examples/wasm$ touch envoy_filter_http_wasm_example.cc && make
  +```
  +
  