add license info

This project says that it's emscripten-compiled zlib, which [has license information](http://zlib.net/zlib_license.html). Can we add license information to this project specifically?

This PR  is a starting point. I don't know if the changes I made here are exactly correct or not. Please either (1) advise me on changes or (2) close this and make the changes yourself.

This is important for people who want to use zpipe in their own projects. Thanks!