Add calibrator make instructions to README

[joeroe]

Needed in a dev environment anyway. Not sure about Docker?

[MartinHinz]

Added documentation, you might like to check or close this issue.

[MartinHinz]

Actually, as we have discussed this, I have outsourced the calibrator to its own docker container, so that we can use it as an API. You can find this repo here: https://github.com/xronos-ch/calibrator_api. But also, since you have developed such a fine tool like Ruby radiocarbon, maybe it also makes sense to switch to this? Depends on the speed!