[core-workflow] Choosing a CI service

[Alexander Belopolsky]

I have recently set up several CI services in my cpython fork on
Github: <https://github.com/abalkin/cpython/tree/ci>.

I am not ready to make any recommendations, but feel free to click on
the build badges at the top of the README file and see the results for
yourself.  I activated the following services:

* Travis CI
* CodeCov
* CodeShip
* AppVeyor  (windows)