Add note how to use environment variables for configuration

README.md

@@ -22,6 +22,7 @@ device which produces mail.
 
 ## Main features
 
+* Simple configuration with ini file or environment variables
 * Supports SMTPS/TLS (465), STARTTLS (587) and unencrypted SMTP (25)
 * Checks for sender, receiver, client IP
 * Authentication support with file (LOGIN, PLAIN)

smtprelay.ini

@@ -1,4 +1,8 @@
 ; smtprelay configuration
+;
+; All config parameters can also be provided as environment
+; variables in uppercase and the prefix "SMTPRELAY_".
+; (eg. SMTPRELAY_LOGFILE, SMTPRELAY_LOG_FORMAT)
 
 ; Logfile (blank/default is stderr)
 ;logfile =