Update documentation for submdule sync

Summary:
Subodule sync and update is necessary whenver commit hash of any submodule
changes. Thus it is safe to do git submodule update anytime you pull from
upstream

Reviewed By: mcr229

Differential Revision: D48357048

fbshipit-source-id: 29622668e56a9aefa0004b33b577898f66ae8841