Add more instuctions on how to publish docker images yourself.

/cc @jozef

@jozef Please let me know if you have any concerns or suggestions.

@sytses looks good.. was actually thinking to update the readme not to include version in image name but use it as a tag.

@jozef I think that would be a great improvement. As for the naming, shall we do gitlab_ce and gitlab_data or gitlab-ce and gitlab-data?

Actually haven't seen anywhere in registry images with underscore.. Ups.. I may prepare update for Readme with version as a tag.. Don't really have too much free time, so it could take 2-3 days.