Document MSRV policy about mio HOT 1 CLOSED

One thing to consider is dependencies and their MSRV bumps.

Tokio's current policy here is that for the latest version, we require all dependencies to have a matching MSRV, and we will remove the dependency or bump the MSRV if a dependency increases it past that.

Tokio's current policy for LTS releases is that we try to do the same, but that we don't guarantee it:

Note that although we try to avoid the situation where a dependency transitively increases the MSRV of Tokio, we do not guarantee that this does not happen. However, every minor release will have some set of versions of dependencies that works with the MSRV of that minor release.

from mio.