document ENV_FILE variable usage

.env.template

@@ -3,6 +3,11 @@
 ##
 ## Be aware that most of these settings will be overridden if they were changed
 ## in the admin interface. Those overrides are stored within DATA_FOLDER/config.json .
+##
+## By default, vaultwarden expects for this file to be named ".env" and located
+## in the current working directory. If this is not the case, the environment
+## variable ENV_FILE can be set to the location of this file prior to starting
+## vaultwarden.
 
 ## Main data folder
 # DATA_FOLDER=data