Broken build instructions / incomplete .env example

[jflatow]

If you follow the directions in the README for how to build the Chrome extension (or any other extension, I imagine), the first issue you will hit is ARGENT_X_ENVIRONMENT is missing. You can copy the .env.example to .env, but then you hit No multisig base url, at which point it becomes very clear that there is non-trivial configuration missing from the documentation in order to build correctly.