Add OpenSSF best practices badge. (#100496)

We are working towards following OpenSSF best practice recommendations
for flutter/flutter. This badge presents our current progress in this
effort.

README.md

@@ -3,6 +3,7 @@
 [![Build Status - Cirrus][]][Build status]
 [![Discord badge][]][Discord instructions]
 [![Twitter handle][]][Twitter badge]
+[![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/5631/badge)](https://bestpractices.coreinfrastructure.org/projects/5631)
 
 Flutter is Google's SDK for crafting beautiful, fast user experiences for
 mobile, web, and desktop from a single codebase. Flutter works with existing