readme: Add OSSF Scorecard badge

While we are not perfect, this gives us a concrete TODO list.

Author: rettichschnidi

README.md

@@ -2,6 +2,8 @@
 
 [![Build](https://github.com/eclipse/wakaama/actions/workflows/build_and_test.yaml/badge.svg)](https://github.com/eclipse/wakaama/actions/workflows/build_and_test.yaml)
 
+[![OpenSSF Scorecard](https://api.scorecard.dev/projects/github.com/eclipse-wakaama/wakaama/badge)](https://scorecard.dev/viewer/?uri=github.com/eclipse-wakaama/wakaama)
+
 Wakaama (formerly liblwm2m) is an implementation of the Open Mobile Alliance's LightWeight M2M
 protocol (LWM2M).