Add lookup option documentation (#1483)

Co-authored-by: Sindre Sorhus <sindresorhus@gmail.com>

Author: Giotino

readme.md

@@ -655,6 +655,15 @@ await got('https://api6.ipify.org', {
 });
 ```
 
+###### lookup
+
+Type: `Function`\
+Default: [`dns.lookup`](https://nodejs.org/api/dns.html#dns_dns_lookup_hostname_options_callback)
+
+Custom DNS resolution logic.
+
+The function signature is the same as [`dns.lookup`](https://nodejs.org/api/dns.html#dns_dns_lookup_hostname_options_callback).
+
 ###### request
 
 Type: `Function`\