Add reasoning to introduction

research/Github Actions vs Jenkins vs CircleCI.md

@@ -1,6 +1,7 @@
 # GitHub Actions vs Jenkins Comparison vs CircleCI
 
-This document describes a comparison between [GitHub Actions](https://github.com/features/actions) and [Jenkins](https://www.jenkins.io/). 
+This document describes a comparison between [GitHub Actions](https://github.com/features/actions) and [Jenkins](https://www.jenkins.io/) and [CircleCI](https://circleci.com/).
+These two CI/CD tools where choose, because they are among the most popular [^35], [^36], [^37].
 
 ## Similarities
 
@@ -252,4 +253,7 @@ offers to host all runners in the cloud, while still providing enough resources
 [^31]: https://medium.com/@afimaamedufie/circling-around-circleci-8a1a15d751ff
 [^32]: https://circleci.com/pricing/price-list/
 [^33]: https://circleci.com/docs/using-matrix-jobs/
-[^34]: https://circleci.com/docs/configuration-reference/#matrix
+[^34]: https://circleci.com/docs/configuration-reference/#matrix
+[^35]: https://thectoclub.com/tools/best-ci-cd-tools/
+[^36]: https://katalon.com/resources-center/blog/ci-cd-tools
+[^37]: https://spacelift.io/blog/ci-cd-tools