Add badge for OpenSSF scorecard (#181)

README.md

@@ -1,6 +1,7 @@
 # smtprelay
 
 [![Go Report Card](https://goreportcard.com/badge/github.com/decke/smtprelay)](https://goreportcard.com/report/github.com/decke/smtprelay)
+[![OpenSSF Scorecard](https://img.shields.io/ossf-scorecard/github.com/decke/smtprelay?label=openssf%20scorecard&style=flat)](https://scorecard.dev/viewer/?uri=github.com/decke/smtprelay)
 
 Simple Golang based SMTP relay/proxy server that accepts mail via SMTP
 and forwards it directly to another SMTP server.