doc: Add CII Best Practices badge to README.md

Node.js has earned the Linux Foundation Core Infrastructure Initiative (CII) best practices badge.
This change lets people see that, including a link for more information.

The badge lets users and potential users of Node.js know that Node.js follows best practices.
It may also help spur other projects to follow Node.js's lead.

My thanks to Rod Vagg, who did the work to see that Node.js earned the badge.

PR-URL: https://github.com/nodejs/node/pull/6819
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>
Reviewed-By: Myles Borins <myles.borins@gmail.com>

README.md

@@ -1,7 +1,7 @@
 Node.js
 =======
 
-[![Gitter](https://badges.gitter.im/Join Chat.svg)](https://gitter.im/nodejs/node?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
+[![Gitter](https://badges.gitter.im/Join Chat.svg)](https://gitter.im/nodejs/node?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge) [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/29/badge)](https://bestpractices.coreinfrastructure.org/projects/29)
 
 Node.js is a JavaScript runtime built on Chrome's V8 JavaScript engine. Node.js
 uses an event-driven, non-blocking I/O model that makes it lightweight and