Update `README.md` regarding environment variables

README.md

@@ -19,6 +19,16 @@ short_description: Spring 2025 CTP Slack Bot RAG system
 
 ## How to Run the Application
 
+You need to configure it first. This is done via environment variables, or an `.env` file based on the template, `.env.template`.
+
+Obtaining the values requires setting up API tokens/secrets with:
+
+* Slack: for `SLACK_BOT_TOKEN` and `SLACK_APP_TOKEN`
+* MongoDB: for `MONGODB_URI`
+* OpenAI: for `OPENAI_API_KEY`
+* Google Drive: for `GOOGLE_PROJECT_ID`, `GOOGLE_CLIENT_ID`, `GOOGLE_CLIENT_EMAIL`, `GOOGLE_PRIVATE_KEY_ID`, and `GOOGLE_PRIVATE_KEY`
+    * For Google Drive, set up a service account. It’s the only supported authentication type.
+
 ### Normally
 
 Just run the Docker image. 😉