Clarify image version scheme

[e-minguez]

It seems to be:

For the version 2.2.1-7.6 2.2.1 is the tag in the rancher/elemental repository, 7 is the source change counter, and 6 is the build counter

so I guess adding this https://elemental.docs.rancher.com/custom-images#finding-elemental-base-images would be nice.