add further environment variable

README.md

 export POSTTLS_ENVIRONMENT_TYPE="development"
 
 # PostTLS settings
+export POSTTLS_NOTIFICATION_SYSADMIN_MAIL_ADDRESS="admin@localhost"
 export POSTTLS_NOTIFICATION_SENDER="postmaster@domain.com (Postmaster)"
 export POSTTLS_NOTIFICATION_SMTP_HOST="localhost"