add OpenSSF Scorecard badge to README

Add OpenSSF Scorecard badge to README. It is checked by the CLOmonitor
and lowers our score by not having it. We have worked to improve the
scores, so we can display the score as well.

Signed-off-by: Tuomo Tanskanen <tuomo.tanskanen@est.tech>

README.md

@@ -1,6 +1,7 @@
 # Metal3 IP Address Manager for Cluster API Provider Metal3
 
 [![CLOMonitor](https://img.shields.io/endpoint?url=https://clomonitor.io/api/projects/cncf/metal3-io/badge)](https://clomonitor.io/projects/cncf/metal3-io)
+[![OpenSSF Scorecard](https://api.securityscorecards.dev/projects/github.com/metal3-io/ip-address-manager/badge)](https://securityscorecards.dev/viewer/?uri=github.com/metal3-io/ip-address-manager)
 [![Ubuntu daily main build status](https://jenkins.nordix.org/buildStatus/icon?job=metal3_daily_main_integration_test_ubuntu&subject=Ubuntu%20daily%20main)](https://jenkins.nordix.org/view/Metal3/job/metal3_daily_main_integration_test_ubuntu/)
 [![CentOS daily main build status](https://jenkins.nordix.org/buildStatus/icon?job=metal3_daily_main_integration_test_centos&subject=CentOS%20daily%20main)](https://jenkins.nordix.org/view/Metal3/job/metal3_daily_main_integration_test_centos/)