Added badge for CII Best Practices.

Thanks to CII and xet7 !

README.md

@@ -12,6 +12,7 @@
 [![Project Dependencies](https://david-dm.org/wekan/wekan.svg "Project Dependencies")](https://david-dm.org/wekan/wekan)
 [![Code analysis at Open Hub](https://img.shields.io/badge/code%20analysis-at%20Open%20Hub-brightgreen.svg "Code analysis at Open Hub")](https://www.openhub.net/p/wekan)
 [![FOSSA Status](https://app.fossa.io/api/projects/git%2Bgithub.com%2Fwekan%2Fwekan.svg?type=shield)](https://app.fossa.io/projects/git%2Bgithub.com%2Fwekan%2Fwekan?ref=badge_shield)
+[![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/4619/badge)](https://bestpractices.coreinfrastructure.org/projects/4619)
 
 ## [Translate Wekan at Transifex](https://transifex.com/wekan/wekan)