Add slack notification example

Author: mwakaba2

README.md

@@ -27,9 +27,13 @@ Option to get notebook cell completion notifications via browser, slack, mobile,
 <p float="left">
     <img width="300" alt="pushover_iphone_example" src="https://user-images.githubusercontent.com/3497137/136384645-843b8496-ad40-4c89-998b-ff46ea9f73a7.png">
     <img width="300" alt="pushover_android_example" src="https://user-images.githubusercontent.com/3497137/136384911-1759a502-b51b-4da2-bbe9-a0244c4de468.png">
-
 </p>
 
+*Image of slack notification via `ntfy + slack_webhook`*
+
+<img width="899" alt="Screen Shot 2021-10-09 at 1 57 51 PM" src="https://user-images.githubusercontent.com/3497137/136669318-4e93e062-be38-4881-b63e-b36f69547a0a.png">
+
+
 ## Quick demos and tutorials :notebook:
 
 To test out this extension without any local set-up, please check out the [binder link](https://mybinder.org/v2/gh/mwakaba2/jupyterlab-notifications/main?urlpath=lab/tree/tutorial/py3_demo.ipynb). This will set-up the environment, install the extension, and take you to several demo notebooks for you to play around with to get familiar with the notifications extension.