docs: add domains.json example and explanation

README.md

@@ -25,3 +25,22 @@ apikey:
 
 ...
 ```
+
+### Adding target domains
+
+Add every domain you want to send an update for to the file `domains.json`.
+The file should list each domain whose global DNS record should point to the local host's public IP.
+The file must have the following format:
+
+```json
+{
+  "domains": [
+    "example.com",
+    "subdomain1.example.com",
+    "subdomain2.example.com",
+  ],
+  "description": "My dynamic DNS."
+}
+```
+
+The description can have any string as a value, but it must be present.

domains.json

@@ -0,0 +1,8 @@
+{
+  "domains": [
+    "example.com",
+    "subdomain1.example.com",
+    "subdomain2.example.com",
+  ],
+  "description": "My dynamic DNS."
+}