docs: add screenshots for options and popup pages in README

Author: cmuench

README.md

@@ -20,6 +20,8 @@ This Firefox extension allows you to manage and trigger webhooks directly from y
 
 ## Managing Webhooks
 
+![Options Page Screenshot](./docs/images/options.png)
+
 ### Add a Webhook
 1. Go to the options page (`Options` in the extension menu).
 2. Enter a name and the webhook URL in the provided fields.
@@ -37,6 +39,8 @@ This Firefox extension allows you to manage and trigger webhooks directly from y
 
 ## Triggering Webhooks
 
+![Popup Page Screenshot](./docs/images/popup.png)
+
 1. Click the extension icon in the Firefox toolbar to open the popup.
 2. Select the webhook you want to trigger from the list.
 3. Click the "Send" or equivalent button to trigger the webhook.