[Support] Remove local dns from must_direct in README

## What would you like us to support (In short summary)

### Motivation & Background

If we add `dnsmasq` and `systemd-resolved` to must_direct, in 'binding to WAN' scenario, DNS mapping will fail because all DNS queries will bypass `dae`.

The best way is to refer to [external-dns.md](https://github.com/daeuniverse/dae/blob/main/docs/getting-started/external-dns.md). But it is not OOTB (out-of-the-box) and is expensive to configure.

Removing them from must_direct is not the best way but just acceptable. Its **disadvantage** is that those local domain servers have cache and they will not always request to upstream each time, thus if some domains share the same IP, domain rules can be incorrectly matched due to incorrect mappings.