Updated README.md to include extra information on configuration of stripe in test environment.

README.md

@@ -68,7 +68,8 @@ copy the file `env.sample` and rename it `.env`. then update its content accordi
   ```
 - `SESSION_SECRET_KEY`: see item above for how to generate a random string
 
-- `STRIPE_SECRET_KEY`: Stripe API key
+- `STRIPE_SECRET_KEY`: Stripe API key for production
+- `STRIPE_TEST_SECRET_KEY`: Stripe API key for dev environment. Put here your TEST API key. Beware that data in test environment differs from production
 - `STRIPE_ENDPOINT_SECRET`: Stripe webhook secret
   - important: use keys generated from Test mode while working on the website