Add Missing serialize Documentation (#1111)

Readme.md

@@ -149,6 +149,7 @@ Does your company use Cheerio in production? Please consider [sponsoring this pr
 <details>
   <summary>Forms</summary>
 
+  - [.serialize()](#serialize)
   - [.serializeArray()](#serializearray)
 </details>
 <details>
@@ -429,6 +430,15 @@ Checks the current list of elements and returns `true` if _any_ of the elements
 
 ### Forms
 
+#### .serialize()
+
+Encodes a set of form elements as a URL query string.
+
+```js
+$('<form><input name="foo" value="bar" checked /><input name="foo" value="qux" checked /></form>').serialize()
+//=> foo=bar&foo=qux
+```
+
 #### .serializeArray()
 
 Encode a set of form elements as an array of names and values.