Documentation on migrating to 3.x major version

[buep]

There needs to be a migration guide on migrating to the coming new major version from earlier versions (latest 2-3 version backwards at most)

Step by step guide in either markdown in the repository, or on the plugin wiki.
It needs to have a footprint on the plugin wiki - linking to markdown if we put it in markdown.

[buep]

Unless #56 is being implemented, we only need to advise and encourage users to move to the new interface.
If #56 is made we need a real good step by step guide.

The estimate is for being backwards compatible.

[buep]

Make sure also the plugin write a warning in the UI, when using job dsl etc.

[buep]

Closing, replaced by better written #96