add documentation about the available image tags

[lachlan-roberts]

Due to the increase in the number of available image tags from PR #77 and #89, our listing on https://hub.docker.com/_/jetty will become very hard to read.

It would be good to have some summary in the documentation about what baseImage / jdk / jetty combinations we support and what tags correspond to them.

[joakime]

Some projects have some insane numbers of tags.
See the end of docker/hub-feedback#238

We will still have to maintain the list, but we can choose to put them somewhere on docker-library/docs and link to that from the published hub.docker.com location. (example: https://hub.docker.com/_/tomcat links to https://github.com/docker-library/docs/blob/master/tomcat/README.md#supported-tags-and-respective-dockerfile-links)