Update README.md to remove update_sub description

It seems update_sub has been removed since v162, need to clear readme to avoid misunderstanding.
Merge pull request #433 from resouer/patch-3

Update README.md to remove update_sub description
Revert "Merge pull request #433 from resouer/patch-3"

This reverts commit 871e001e49c6ee26db585ff429633510bc28c5ad, reversing
changes made to c4dfff2fe703fd05c4a9044b492d8e4abfb4ac6b.