doc: add `added:` information for v8

PR-URL: https://github.com/nodejs/node/pull/6684
Ref: https://github.com/nodejs/node/issues/6578
Reviewed-By: Ben Noorhduis <info@bnoordhuis.nl>
Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/v8.md

@@ -7,6 +7,9 @@ built with Node.js.  These interfaces are subject to change by upstream and are
 therefore not covered under the stability index.
 
 ## v8.getHeapStatistics()
+<!-- YAML
+added: v1.0.0
+-->
 
 Returns an object with the following properties
 
@@ -22,6 +25,9 @@ Returns an object with the following properties
 ```
 
 ## v8.getHeapSpaceStatistics()
+<!-- YAML
+added: v6.0.0
+-->
 
 Returns statistics about the V8 heap spaces, i.e. the segments which make up
 the V8 heap. Order of heap spaces nor availability of a heap space can be
@@ -71,6 +77,9 @@ Example result:
 ```
 
 ## v8.setFlagsFromString(string)
+<!-- YAML
+added: v1.0.0
+-->
 
 Set additional V8 command line flags.  Use with care; changing settings
 after the VM has started may result in unpredictable behavior, including