Lack of document migrate from kv_unstable to kv

[tisonkun]

I'm doing the upgrade here fast/logforth#85 and several private codebase.

Although it seems as long as the applications use only log::debug!(val:?; "...") syntax, either kv_unstable or kv should work, I'd prefer an upstream upgrade document or several notes, to confirm that it's fully compatible, or there are esome known issues.

[Thomasdezeeuw]

If you used the latest version of log that supported kv_unstable, then there are not changes when switching to stable kv.