Add CII best practices badge (#1503)

Author: acao

README.md

@@ -12,6 +12,8 @@
 [![Snyk Vulnerabilities for GitHub Repo](https://img.shields.io/snyk/vulnerabilities/github/graphql/graphiql)](https://snyk.io/test/github/graphql/graphiql)
 ![LGTM Grade](https://img.shields.io/lgtm/grade/javascript/github/graphql/graphiql)
 ![LGTM Alerts](https://img.shields.io/lgtm/alerts/github/graphql/graphiql)
+[![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/3887/badge)](https://bestpractices.coreinfrastructure.org/projects/3887)
+or by embedding this in your HTML:
 
 ## Overview