add link to env blogpost

Author: flexbox

challenges/release/02.md

@@ -4,7 +4,7 @@
 
 - [Difference between development builds](https://blog.expo.dev/development-builds-are-out-of-preview-7e5b1979f84b) with `expo-dev-client` and Expo Go.
 - Configuring development and production variants in a managed project.
-- Using `.env` files to manage your environment variables.
+- Using `.env` files to [manage your environment variables](https://expo.dev/blog/what-are-environment-variables).
 
 ## 👾 Before we start the exercise