docs(readme): add configuration notice split-dns #389

[affeldt28]

Adds a configuration notice to the readme.

If you configured your internal dns to point to an internal ip, to also use your services within your network. Ddns-updater will check against this ip and triggers an update to your configured provider. Due to the fact that this record will never be your public ip, this update will be done every time it checks if your ip is up to date.
Adding a public dns to your container will prevent this behavior, because then ddns-updater will not check against your internal dns.

[qdm12]

First, thank you for the PR 👍
Second sorry for the massive delay!!
Third, in 464a557 I added documentation in the Readme for the variable RESOLVER_ADDRESS - here since January but undocumented weirdly - and linked to your issue #389 and that should work such that only the domain name lookup mechanism uses the DNS specified instead of the whole container, which is a bit better.
By default it still uses the Docker network DNS server, but it can be changed for example to 1.1.1.1:53.