update readme for notification feature

Author: pchang388

README.md

@@ -492,21 +492,23 @@ minio:
 | `keep_last` | `int` | `false` | Optional (default: `0`), if exporter can delete older archives in minio.<br>- set to `1+` if you want to retain a certain number of archives<br>-  `0` will result in no action done |
 
 ## Notifications
-It is possible to also send notifications when an export run fails. Currently, the only supported notification service is [apprise](https://github.com/caronc/apprise).
+It is possible to send notifications when an export run fails. Currently, the only supported notification service is [apprise](https://github.com/caronc/apprise). Apprise is a general purpose notification service and has a variety of integrations and includes generic HTTP POST.
 
 ### apprise
 The apprise configuration is a part of the configuration yaml file and can be modified under `notifications.apprise`.
 
-| Item | Type | Required | Description |
-| ---- | ---- | -------- | ----------- |
-| `apprise.service_urls` | `List<str>` | Provide the apprise urls for apprise to send notifications to |
+| Item | Type | Description |
+| ---- | ---- | ----------- |
+| `apprise.service_urls` | `List<str>` | Provide the apprise urls for apprise to send notifications to. Can also be provided as environment variable: `APPRISE_URLS`, see example further below. |
 | `apprise.config_path` | `str` | If specified, overrides `apprise.service_urls`. Can specify the path to an apprise configuration file |
 | `apprise.plugin_paths` | `List<str>` | Provide the plugin paths for apprise to use |
 | `apprise.storage_path` | `str` | For persistent storage, specify a path for apprise to use |
 | `apprise.custom_title` | `str` | Replace the default message title for apprise notifications |
 | `apprise.custom_attachment_path` | `str` | To include a custom attachment to the apprise notification, specify the path to a file | 
 
-`apprise.service_urls` can contain sensitive information and can be specified as an environment variable instead, example: `export APPRISE='["json://localhost:8080/notify"]'`. 
+`apprise.service_urls` can contain sensitive information and can be specified as an environment variable instead as a string list, example: `export APPRISE_URLS='["json://localhost:8080/notify"]'`.
+
+**If using apprise for notifications, one of `apprise.service_urls` or `apprise.config_path` should be specified.**
 
 ## Potential Breaking Upgrades
 Below are versions that have major changes to the way configuration or exporter runs.