Provide instructions on how to use the Image Puller plug-in to improve Che/CRW performance.

Check with rick_wagner whether existing docs at:

https://www.eclipse.org/che/docs/che-7/administration-guide/caching-images-for-faster-workspace-start/

https://www.eclipse.org/che/docs/che-7/administration-guide/defining-the-list-of-images-to-pull/ 

are sufficient, should be improved, or just need to be better surfaced.