Add CircleCI

README.md

 ## Continuous Integration
 *Libraries and applications for continuous integration.*
 
+* [CircleCI](https://circleci.com) - A continuous integration platform.
 * [GitlabCi](https://about.gitlab.com/gitlab-ci/) - Let GitLab CI test, build, deploy your code. TravisCi like.
 * [Jenkins](https://jenkins.io/index.html) - A continous integration platform with [PHP support](http://jenkins-php.org/index.html).
 * [JoliCi](https://github.com/jolicode/JoliCi) - A continuous integration client written in PHP and powered by Docker.