Document environment variables

[hola-soy-milk]

Looking at the blog post on sat: https://www.wasm.builders/suborbital/a-simple-data-hashing-serverless-function-using-sat-3fn0

Sat can be configured to automatically acquire a TLS certificate using LetsEncrypt, so if you want to deploy this service on the public internet, you can add SAT_DOMAIN=example.com as well for automatic HTTPS!

This is really cool! I wonder if we can document this and SAT_HTTP_PORT?