document the prefix method

Author: rushmorem

README.md

@@ -14,6 +14,7 @@ use addr::{parse_domain_name, parse_dns_name};
 let domain = parse_domain_name("www.example.com")?;
 assert_eq!(domain.root(), Some("example.com"));
 assert_eq!(domain.suffix(), "com");
+assert_eq!(domain.prefix(), Some("www"));
 
 let domain = parse_domain_name("www.食狮.中国")?;
 assert_eq!(domain.root(), Some("食狮.中国"));

tests/integration.rs

@@ -61,7 +61,10 @@ fn addr_parsing() {
         });
 
         ctx.it("should allow sub-domains", |_| {
-            assert!(List.parse_domain_name("www.example.com.").is_ok())
+            let domain = List.parse_domain_name("www.example.com.").unwrap();
+            assert_eq!(domain.suffix(), "com.");
+            assert_eq!(domain.root(), Some("example.com."));
+            assert_eq!(domain.prefix(), Some("www"));
         });
 
         ctx.it("should allow IDNs", |_| {