readme: add OpenSSF best practices badge

Author: ethomson

README.md

@@ -1,5 +1,6 @@
 libgit2 - the Git linkable library
 ==================================
+[![OpenSSF Best Practices](https://www.bestpractices.dev/projects/9609/badge)](https://www.bestpractices.dev/projects/9609)
 
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