[misc] Add OpenSSF Scorecard badge to README. (flutter#6376)

Author: ditman

README.md

@@ -2,6 +2,7 @@
 
 [![Build Status](https://api.cirrus-ci.com/github/flutter/plugins.svg)](https://cirrus-ci.com/github/flutter/plugins/main)
 [![Release Status](https://github.com/flutter/plugins/actions/workflows/release.yml/badge.svg)](https://github.com/flutter/plugins/actions/workflows/release.yml)
+[![OpenSSF Scorecard](https://api.securityscorecards.dev/projects/github.com/flutter/plugins/badge)](https://api.securityscorecards.dev/projects/github.com/flutter/plugins)
 
 This repo is a companion repo to the main [flutter
 repo](https://github.com/flutter/flutter). It contains the source code for