docs: document PORT environment variable (#165)

Author: morremeyer

README.md

@@ -24,6 +24,7 @@ The backend can be configured with the following environment variables. None are
 | Name                 | Type                      | Default                                              | Description                                                                                                                                                       |
 | -------------------- | ------------------------- | ---------------------------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------- |
 | `GIN_MODE`           | One of `release`, `debug` | `release`                                            | The mode that gin runs in. Only set this to `debug` on your development environment!                                                                              |
+| `PORT`               | `number`                  | `8080`                                               | The port the backend listens on                                                                                                                                   |
 | `LOG_FORMAT`         | One of `json`, `human`    | `json` if `GIN_MODE` is `release`, otherwise `human` | If log output is written human readable or as JSON.                                                                                                               |
 | `DB_HOST`            | `string`                  |                                                      | hostname or address of postgresql                                                                                                                                 |
 | `DB_USER`            | `string`                  |                                                      | username for the postgresql connection                                                                                                                            |