Document DB URL in .env file

.env.template

@@ -4,8 +4,13 @@
 ## Main data folder
 # DATA_FOLDER=data
 
-## Individual folders, these override %DATA_FOLDER%
+## Database URL
+## When using SQLite, this is the path to the DB file, default to %DATA_FOLDER%/db.sqlite3
+## When using MySQL, this it is the URL to the DB, including username and password:
+## Format: mysql://[user[:password]@]host/database_name
 # DATABASE_URL=data/db.sqlite3
+
+## Individual folders, these override %DATA_FOLDER%
 # RSA_KEY_FILENAME=data/rsa_key
 # ICON_CACHE_FOLDER=data/icon_cache
 # ATTACHMENTS_FOLDER=data/attachments