feat(env): add example environment file and update .gitignore to include .env

Author: fulleni

.env.example

@@ -0,0 +1,5 @@
+# This is an example environment file.
+# Copy this file to .env and fill in your actual configuration values.
+# The .env file is ignored by Git and should NOT be committed.
+
+DATABASE_URL="postgres://user:password@localhost:5432/db_name"

.gitignore

@@ -13,4 +13,7 @@ build/
 .dart_frog
 
 # Test related files
-coverage/
+coverage/
+
+# Environment variables
+.env