docs: update readme with URL support (#146)

v6.0.0 introduced URL support, this PR updates the readme to document that change.

README.md

@@ -47,13 +47,14 @@ serialize({
     fn   : function echo(arg) { return arg; },
     re   : /([^\s]+)/g,
     big  : BigInt(10),
+    url  : new URL('https://example.com/'),
 });
 ```
 
 The above will produce the following string output:
 
 ```js
-'{"str":"string","num":0,"obj":{"foo":"foo"},"arr":[1,2,3],"bool":true,"nil":null,"undef":undefined,"inf":Infinity,"date":new Date("2016-04-28T22:02:17.000Z"),"map":new Map([["hello","world"]]),"set":new Set([123,456]),"fn":function echo(arg) { return arg; },"re":new RegExp("([^\\\\s]+)", "g"),"big":BigInt("10")}'
+'{"str":"string","num":0,"obj":{"foo":"foo"},"arr":[1,2,3],"bool":true,"nil":null,"undef":undefined,"inf":Infinity,"date":new Date("2016-04-28T22:02:17.000Z"),"map":new Map([["hello","world"]]),"set":new Set([123,456]),"fn":function echo(arg) { return arg; },"re":new RegExp("([^\\\\s]+)", "g"),"big":BigInt("10"),"url":new URL("https://example.com/")}'
 ```
 
 Note: to produce a beautified string, you can pass an optional second argument to `serialize()` to define the number of spaces to be used for the indentation.