Add an intro to CI services documentation [ci skip]

doc/ci/services/README.md

 ## GitLab CI Services
 
+GitLab CI uses the `service` keyword to define what docker containers should be
+linked with your base image. Below is a list of examples you may use.
+
 + [Using MySQL](mysql.md)
 + [Using PostgreSQL](postgres.md)
 + [Using Redis](redis.md)
-+ [Using Other Services](../docker/using_docker_images.html#how-to-use-other-images-as-services)
++ [Using Other Services](../docker/using_docker_images.md#how-to-use-other-images-as-services)