Correct the readme note about zvol snapshots

[Shados]

You can snapshot a zvol that occupies 85% of your pool, you just need to change its refreservation to a normal reservation (or remove its reservation entirely, making it a sparse zvol). Both of those have potential risks / consequences in turn, of course, so read the ZFS man page entries for them carefully and give it some thought before doing so.

ZFS uses refreservation by default for zvols because otherwise in a free-space contention scenario you could accidentally run out of space for a zvol due to snapshots, despite thinking you had sufficient space reserved from the beginning. refreservation reserves space in the pool for the dataset without allowing children of that dataset (relevantly here, including snapshots) to consume from that reservation, while a normal reservation will be consumed by child datasets.