1746: DNS records for client autoconfiguration (RFC6186) r=Diman0 a=nextgens
## What type of PR?
Feature
## What does this PR do?
Add instructions on how to configure rfc6186 DNS records for client autoconfiguration
### Related issue(s)
- #224
- #498
## Prerequistes
Before we can consider review and merge, please make sure the following list is done and checked.
If an entry in not applicable, you can check it or remove it from the list.
- [x ] Unless it's docs or a minor change: add [changelog](https://mailu.io/master/contributors/guide.html#changelog) entry file.
Co-authored-by: Florent Daigniere <nextgens@freenetproject.org>
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