added readme links

packages/medusa-plugin-slack-notification/README.md

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+# medusa-plugin-slack-notification
+
+Slack Integration to receive new order notifications on Slack.
+
+Learn more about how you can use this plugin in the [documentaion](https://docs.medusajs.com/add-plugins/slack).
+
+## Options
+
+```js
+{
+  show_discount_code: false,
+  slack_url: `<WEBHOOK_URL>`,
+  admin_orders_url: `http://localhost:7001/a/orders`
+}
+```