README.md: CII best practices badge

README.md

-[![Travis Build Status](https://travis-ci.org/warmcat/libwebsockets.svg)](https://travis-ci.org/warmcat/libwebsockets) [![Appveyor Build status](https://ci.appveyor.com/api/projects/status/qfasji8mnfnd2r8t?svg=true)](https://ci.appveyor.com/project/lws-team/libwebsockets) [![Coverity Scan Build Status](https://scan.coverity.com/projects/3576/badge.svg)](https://scan.coverity.com/projects/3576)
+[![Travis Build Status](https://travis-ci.org/warmcat/libwebsockets.svg)](https://travis-ci.org/warmcat/libwebsockets) [![Appveyor Build status](https://ci.appveyor.com/api/projects/status/qfasji8mnfnd2r8t?svg=true)](https://ci.appveyor.com/project/lws-team/libwebsockets) [![Coverity Scan Build Status](https://scan.coverity.com/projects/3576/badge.svg)](https://scan.coverity.com/projects/3576) [![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/2266/badge)](https://bestpractices.coreinfrastructure.org/projects/2266)
 
 ![overview](./doc-assets/lws-overview.svg)