Document HTTP retrieval env vars

## background
https://github.com/ipfs/rainbow/pull/242/files adds http retrieval and introduces a number of configurable env vars:

```
ROUTING_IGNORE_PROVIDERS
RAINBOW_HTTP_RETRIEVAL_ENABLE
RAINBOW_HTTP_RETRIEVAL_ALLOWLIST
RAINBOW_HTTP_RETRIEVAL_WORKERS
```

We should document these in https://github.com/ipfs/rainbow/blob/main/docs/environment-variables.md