doc: add `added:` info for `process.cpuUsage`

Refs: https://github.com/nodejs/node/issues/6578
PR-URL: https://github.com/nodejs/node/pull/6863
Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/process.md

@@ -547,7 +547,9 @@ added: v0.7.2
 If `process.connected` is `false`, it is no longer possible to send messages.
 
 ## process.cpuUsage([previousValue])
-<!-- TODO add YAML block when cpuUsage is in a release -->
+<!-- YAML
+added: v6.1.0
+-->
 
 Returns the user and system CPU time usage of the current process, in an object
 with properties `user` and `system`, whose values are microsecond values