document the fact zstd support is currently halfway removed

[flokli]

The README of the main branch advertises zstd compression, but #1426 removed it entirely.

While the plan is to reintroduce this on top of marble, semantics on the main branch are currently not very ideal (and it's been like this for a year).

Could you add a big fat warning on top of the README describing the current state of things, so people don't actually use it in this state?