[readme] add CII best practices badge

Author: ljharb

README.md

@@ -8,6 +8,7 @@
 [![coverage][codecov-image]][codecov-url]
 [![License][license-image]][license-url]
 [![Downloads][downloads-image]][downloads-url]
+[![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/9058/badge)](https://bestpractices.coreinfrastructure.org/projects/9058)
 
 [![npm badge][npm-badge-png]][package-url]