"Using 3rd-Party Libraries" guide added
"Using 3rd-Party Libraries" guide updated
Missing "md" extension added for "Using 3rd-Party Libraries" guide part
Merge branch 'master' of github.com:yiisoft/yii2 into 1837-3rd-party-doc
Merge PR #1981 branch '1837-3rd-party-doc' of https://github.com/klimov-paul/yii2

* '1837-3rd-party-doc' of https://github.com/klimov-paul/yii2:
  Missing "md" extension added for "Using 3rd-Party Libraries" guide part
  "Using 3rd-Party Libraries" guide updated
  "Using 3rd-Party Libraries" guide added
linked guide on 3rd party systems