README.adoc: stop mentioning the removed CentOS Dockerfile

Signed-off-by: Michael Opdenacker <[email protected]>

README.adoc

@@ -360,11 +360,10 @@ where all repositories are found.
 Docker files are provided in the `docker/` directory. To generate a Docker image
 for indexing the sources of a project, first fetch the Elixir repository. Then
 pick the project you want to index (Musl is nice test data being not too slow
-to index) and your target distribution (`debian` and `centos` available), and
+to index) and your target distribution (`debian` currently), and
 run:
 
  $ docker build -t elixir-debian --build-arg GIT_REPO_URL=git://git.musl-libc.org/musl --build-arg PROJECT=musl ./docker/debian/
- $ docker build -t elixir-centos --build-arg GIT_REPO_URL=git://git.musl-libc.org/musl --build-arg PROJECT=musl ./docker/centos/
 
 Then you can use your new container as follows (you get the container id from the output of `docker build`):