Added united-domains reselling DNS API

dnsapi.md

@@ -2838,6 +2838,28 @@ S-dns nameservers use anycast. It is therefore possible that the server next to
 acme.sh --issue --dns dns_sdns --domain example.com --dnssleep 240
 ```
 
+## 141. Using the united-domains reselling DNS API
+
+Create an account at [ud reselling](https://www.ud-reselling.com/).
+
+There is no option to use an API key, so you just need to set your credentials as environment variables:
+
+```sh
+export UDR_USER="..."
+export UDR_PASS="..."
+```
+
+To issue a certificate, execute:
+```sh
+acme.sh --issue --dns dns_udr -d example.com -d www.example.com
+```
+
+To issue a wildcard certificate, execute:
+```sh
+acme.sh --issue --dns dns_udr -d example.com -d *.example.com
+```
+
+The `UDR_USER` and `UDR_PASS` will be saved in `~/.acme.sh/account.conf` and will be reused when needed.
 
 # ##################
 # Use custom API