doc: add added: information for querystring

Module introduced in 7ff04c1.

Ref: nodejs#6578

doc/api/querystring.md

@@ -8,11 +8,17 @@ This module provides utilities for dealing with query strings.
 It provides the following methods:
 
 ## querystring.escape
+<!-- YAML
+added: v0.1.25
+-->
 
 The escape function used by `querystring.stringify`,
 provided so that it could be overridden if necessary.
 
 ## querystring.parse(str[, sep][, eq][, options])
+<!-- YAML
+added: v0.1.25
+-->
 
 Deserialize a query string to an object.
 Optionally override the default separator (`'&'`) and assignment (`'='`)
@@ -38,6 +44,9 @@ querystring.parse('w=%D6%D0%CE%C4&foo=bar', null, null,
 ```
 
 ## querystring.stringify(obj[, sep][, eq][, options])
+<!-- YAML
+added: v0.1.25
+-->
 
 Serialize an object to a query string.
 Optionally override the default separator (`'&'`) and assignment (`'='`)
@@ -63,6 +72,9 @@ querystring.stringify({ w: '中文', foo: 'bar' }, null, null,
 ```
 
 ## querystring.unescape
+<!-- YAML
+added: v0.1.25
+-->
 
 The unescape function used by `querystring.parse`,
 provided so that it could be overridden if necessary.