Come up with policy on adding support for each Core release

[tcharding]

We probably should come up with some sort of policy on when we add support for each version of Core. I don't know the exact Core release process but I imagine there is some structure e.g a release candidate some amount of time before the release

1. Aim to have a draft up while a new version is being developed
2. Do it during their RC cycle (or whatever they do)
3. Do it within some number of weeks after the release
4. Do it whenever we feel like it

I'm leaning towards (2) - how do you feel about it @jamillambert

(I'll find out their release process at some stage.)

[jamillambert]

I agree during their RC cycle would be good. We need to have some notification method or to keep an eye on it and open an issue here as soon as possible, so they aren't missed.

[jamillambert]

As mentioned in #387 this includes what the policy is and how to handle minor releases of all versions from 17 up.