Add tips for mail.password parameter
Add tips for mail.password parameter

Sometimes, we got an inquiry about this warning. > `Unsecure 'password' parameter is deprecated.`

For example, https://github.com/treasure-data/digdag/issues/356
treasure-data/digdag,treasure-data/digdag,treasure-data/digdag,treasure-data/digdag,treasure-data/digdag,treasure-data/digdag,treasure-data/digdag