Add license files

Hello! I was looking over the repo and realized that there aren't any license files:

The witx tool has a license listed in the cargo.toml, but we should include a license file as well.
I'd like to add a license for the entire repo if possible, but especially for the docs and witx data files themselves.

I'm not aware of if there's any formal process required for this or if everything in the WebAssembly org is assumed to be under a certain license.

Apache 2 seems reasonable to me though -- it's what witx and some other projects in the WebAssembly org are using. If that sounds reasonable to you all, I can make a pull request to add it!

edit:

looks like there's been a need to use a separate documentation license in the past https://github.com/WebAssembly/spec/pull/496. That raises the question of whether witx is documentation or source code...

edit2: not sure how I missed the license file on witx -- thanks for the correction!