book: document how to run hickory-dns (binary) as a recursive resolver

[japaric]

as far as I could tell the steps are:

1. cargo install hickory-dns --features recursor

2. create /etc/named.toml with the contents

[[zones]]
zone = "."
zone_type = "Hint"
stores = { type = "recursor", roots = "/etc/root.hints" }

In production, /etc/root.hints should be fetched from https://www.iana.org/domains/root/files

3. Run hickory-dns

4. Check with dig @localhost A www.example.com.

It should be noted that, currently, DNSSEC validation is disabled and cannot be enabled.

---

if that sounds about right, I can submit a PR with the above information.

maybe some information about _cache_size should be included.

same for zone_type. should it ever be set to some other variant for this use case?

[bluejekyll]

That looks about right. I did want to start spending some time laying out the book for giving direction like this.