Add mention to version being cache in the resource

[daniellavoie]

With git driver, manually updating the version value from the version file
does not magically update the next version that will be used since the
ongoing version is cached in the concourse resource.

[vito]

Thanks for submitting this! Sorry for the delay in review.

This is actually true of all drivers, not just git. The semantics are a bit nuanced than just a cache though: if a version is updated out-of-band, that doesn't change the fact that prior versions existed and were used for builds, so they're actually preserved so that all that metadata doesn't go away. Maybe with concourse/rfcs#1 we could mark them 'deleted' somehow, though that's kind of tricky...

Maybe we could document something saying that when using this resource, the history in the pipeline is the "source of truth", and the driver is only used for persisting the current version so that it's preserved when the pipeline is moved elsewhere or recreated. It's been a long time since you submitted this, so if you're no longer interested in updating the README to that affect, I don't blame ya.

The semantics (ha) of this resource may change even more with concourse/concourse#1707.