Add Tag naming convention to README.md / Documentation

[JanoschDeurer]

Steps to reproduce

Open https://github.com/jenkinsci/docker or https://hub.docker.com/r/jenkins/jenkins in your browser

Actual result

On https://hub.docker.com/r/jenkins/jenkins it is described, that jenkins/jenkins:lts and jenkins/jenkins exits. There is however no explanation on what these tags reference, and what naming convention is used for the tags.

Expected outcome

Other images like the official node image provide a section of tags that show which tag is currently mapped to which. Also a declaration of the release cycle would be great.

In a quick search I found two tickets that also raised questions about this tag naming convention (#265, #700) so it wold be great to have documentation for this :)

[chenrui333]

that is good idea!