Make button vector

Author: mattvagni

SETUP.md

@@ -3,7 +3,7 @@
 ## 1. Install the Slack app
 
 <a href="https://slack.com/oauth/v2/authorize?scope=channels%3Ahistory%2Cchat%3Awrite%2Ccommands%2Cusers%3Aread%2Cusers%3Aread.email&amp;user_scope=&amp;redirect_uri=https%3A%2F%2Fplain-slack-integration-production.onrender.com%2Fslack%2Foauth_redirect&amp;client_id=5201178479941.5226688564182" target="_blank">
-<img src="./img/add_to_slack_button.png">
+<img src="./img/add_to_slack_button.svg">
 </a>
 
 ## 2. Create an API key