add concept-autoloading.md -wip
Merge remote-tracking branch 'upstream/master' into upstream

* upstream/master:
  Reverted #4098.
  fix duplicated white space and typo [skip ci]
  typo fix
  Fixes #4098: `yii\db\Query::addSelect()` should include `*` when it is called the first time
add concept-autoloading.md