feat: update .env.sample for clarity (#44)

Author: mwbrooks

.env.sample

@@ -1,5 +1,9 @@
-SLACK_APP_TOKEN=YOUR_SLACK_APP_TOKEN
-SLACK_BOT_TOKEN=YOUR_SLACK_BOT_TOKEN
+# Optional, uncomment and set when running without the Slack CLI (python3 app.py).
+# SLACK_APP_TOKEN=YOUR_SLACK_APP_TOKEN
+# SLACK_BOT_TOKEN=YOUR_SLACK_BOT_TOKEN
+
+# Optional, uncomment and set when using a custom Slack instance.
 # SLACK_API_URL=YOUR_SLACK_API_URL
-# This template uses OpenAI, but you can use any other provider!
+
+# Required, set your OpenAI API key.
 OPENAI_API_KEY=YOUR_OPENAI_API_KEY