docs(readme): update description of env variables

README.md

@@ -59,6 +59,7 @@ You have now installed and deployed the Smartlook relay proxy. You can check if
 | `ERROR_LOG_LEVEL`     | `string` | `error`                         | [Nginx error log level](https://nginx.org/en/docs/ngx_core_module.html#error_log). One of `debug`, `info`, `notice`, `warn`, `error`, `crit`, `alert`, `emerg`. | Can be overridden                                                                          |
 | `CLIENT_MAX_BODY_SIZE`     | `string` | `20m`                         | [Nginx client_max_body_size](https://nginx.org/en/docs/http/ngx_http_core_module.html#client_max_body_size). | Can be overridden                                                                          |
 | `PROXY_PORT`          | `number` | `8000`                          | Port which will the HTTP server listen on                                                                                                                       | Can be overridden                                                                          |
+| `LISTEN_IPV6`          | `boolean` | `false` (unset)                           | If set to `true`, the server will also listen on IPv6. The port will be the value of `PROXY_PORT` environment variable                                                                                                                        | Can be overridden                                                                          |
 | `WEB_SDK_HOST`        | `string` | `web-sdk.smartlook.com`         | Smartlook Web SDK host                                                                                                                                          | Only edit this value if using a region other than EU, or if instructed by customer support |
 | `MANAGER_HOST`        | `string` | `manager.eu.smartlook.cloud`    | Smartlook Manager host                                                                                                                                          | Only edit this value if using a region other than EU, or if instructed by customer support |
 | `WEB_SDK_WRITER_HOST` | `string` | `web-writer.eu.smartlook.cloud` | Smartlook Web Writer host                                                                                                                                       | Only edit this value if using a region other than EU, or if instructed by customer support |