README: added "CII best practices" badge

README.md

@@ -4,6 +4,7 @@ c-ares
 [![Build Status](https://travis-ci.org/c-ares/c-ares.svg?branch=master)](https://travis-ci.org/c-ares/c-ares)
 [![Windows Build Status](https://ci.appveyor.com/api/projects/status/03i7151772eq3wn3/branch/master?svg=true)](https://ci.appveyor.com/project/c-ares/c-ares)
 [![Coverage Status](https://coveralls.io/repos/c-ares/c-ares/badge.svg?branch=master&service=github)](https://coveralls.io/github/c-ares/c-ares?branch=master)
+[![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/289/badge)](https://best practices.coreinfrastructure.org/projects/289)
 
 This is c-ares, an asynchronous resolver library.  It is intended for
 applications which need to perform DNS queries without blocking, or need to