doc: improve README

README.md

-Dockerfiles to use for the tests in cubicweb, or in cubes.  
-If you want to deploy cubicweb, you are looking for another repo [docker-cubicweb](https://forge.extranet.logilab.fr/cubicweb/docker-cubicweb).
+# CI Dockerfiles for CubicWeb
+
+These Dockerfiles are used to build the images needed to run the
+steps of the pipelines of the CubiWeb project and all the cubes.
+
+If you want an image to deploy CubicWeb, you are looking for
+the repository [docker-cubicweb](https://forge.extranet.logilab.fr/cubicweb/docker-cubicweb) and its related container registry.
 
 Images: