Add OpenSSF Best Practice Badge (llvm#77398)

This tracks our progress implementing the OpenSSF Best Practices for
LLVM.

Author: tstellar

README.md

@@ -1,6 +1,7 @@
 # The LLVM Compiler Infrastructure
 
 [![OpenSSF Scorecard](https://api.securityscorecards.dev/projects/github.com/llvm/llvm-project/badge)](https://securityscorecards.dev/viewer/?uri=github.com/llvm/llvm-project)
+[![OpenSSF Best Practices](https://www.bestpractices.dev/projects/8273/badge)](https://www.bestpractices.dev/projects/8273)
 [![libc++](https://github.com/llvm/llvm-project/actions/workflows/libcxx-build-and-test.yaml/badge.svg?branch=main&event=schedule)](https://github.com/llvm/llvm-project/actions/workflows/libcxx-build-and-test.yaml?query=event%3Aschedule)
 
 Welcome to the LLVM project!