Enhance the README.md

[visualphoenix]

The current README.md file is pretty lacking. I threw it together. Maybe someone can write one which is better.

[maxadorable]

Let me help

[visualphoenix]

Looking forward to it! Thanks!

[visualphoenix]

Please reference the demo project which shows an example of how to use this library here: https://github.com/visualphoenix/docker-jenkins-demo

[visualphoenix]

We also need to explain the intentional omission of the threaded style shown in the official jenkins docker demo here https://github.com/jenkinsci/docker/blob/alpine/init.groovy

While the threaded init style could be used in conjunction with this repository, I intentionally do not do this. The downside to threading the init of multiple plugins is:

1. the jenkins server can be open and unsecured briefly before the various scripts wake up and initialize
2. multiple threaded access to Jenkins.instance competing to call instance.save() seems slower.

[visualphoenix]

We also can explain how omitting the threaded initialization means we dont need to tell jenkins to quiet down while initializing.

The instance isn't fully accessible to start running builds until after the groovy init script has finished running.

If you have multiple groovy scripts to execute before initialization completes and each script runs instance.save() it is very slow to finish initializing.

Using the threaded plugin init style causes the instance to become available faster, but then the instance becomes available before all of the plugins are initialized. Builds could begin if the uses doesn't request the jenkins instance to quiet down.