Env: Added storage type to default example env

Provides greater consideration to the storage type used and the fact
that it'll place images in public space by default.

.env.example

@@ -26,6 +26,13 @@ DB_DATABASE=database_database
 DB_USERNAME=database_username
 DB_PASSWORD=database_user_password
 
+# Storage system to use
+# By default files are stored on the local filesystem, with images being placed in
+# public web space so they can be efficiently served directly by the web-server.
+# For other options with different security levels & considerations, refer to:
+# https://www.bookstackapp.com/docs/admin/upload-config/
+STORAGE_TYPE=local
+
 # Mail system to use
 # Can be 'smtp' or 'sendmail'
 MAIL_DRIVER=smtp