docs: added openssf scorecard badge

Author: stefanb

README.md

@@ -6,6 +6,7 @@
 [![CodeQL](https://github.com/stefanb/osmshortlink-go/actions/workflows/codeql.yml/badge.svg)](https://github.com/stefanb/osmshortlink-go/actions/workflows/codeql.yml)
 [![Go Report Card](https://goreportcard.com/badge/github.com/stefanb/osmshortlink-go)](https://goreportcard.com/report/github.com/stefanb/osmshortlink-go)
 [![codebeat badge](https://codebeat.co/badges/0dcfa9c5-a59b-46ed-b0a6-30e1bbda9a7e)](https://codebeat.co/projects/github-com-stefanb-osmshortlink-go-main)
+[![OpenSSF Scorecard](https://api.securityscorecards.dev/projects/github.com/stefanb/osmshortlink-go/badge)](https://securityscorecards.dev/viewer/?uri=github.com/stefanb/osmshortlink-go)
 
 Specification: https://wiki.openstreetmap.org/wiki/Shortlink