doc: add `added:` info for `string_decoder`

PR-URL: https://github.com/nodejs/node/pull/6741
Ref: https://github.com/nodejs/node/issues/6578
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Roman Klauke <romaaan.git@gmail.com>

doc/api/string_decoder.md

@@ -18,13 +18,22 @@ console.log(decoder.write(euro));
 ```
 
 ## Class: StringDecoder
+<!-- YAML
+added: v0.1.99
+-->
 
 Accepts a single argument, `encoding` which defaults to `'utf8'`.
 
 ### decoder.end()
+<!-- YAML
+added: v0.9.3
+-->
 
 Returns any trailing bytes that were left in the buffer.
 
 ### decoder.write(buffer)
+<!-- YAML
+added: v0.1.99
+-->
 
 Returns a decoded string.