Document requirements for web / wasm usage.

[nullrocket]

Need to document the requirements for using on the web, requires Cross-Origin-Embedder-Policy to be enabled since it uses SharedArrayBuffers to coordinate indexedb access. The web demo works around this by using COI service worker to add the headers on github pages. Using COI is not solution for production, just an easy way host for demo purposes.

Also need to document the build / code gen requirement.