Add documentation regarding automatic LLVM submodule updates
Add documentation regarding automatic LLVM submodule updates

We are experimenting with/transitioning to automatically updating the LLVM SHA each time LLVM is updated in Google's source repository.

PiperOrigin-RevId: 313697751

iree-org/iree,google/iree,google/iree,google/iree,iree-org/iree,google/iree,google/iree,google/iree,iree-org/iree,iree-org/iree,iree-org/iree,iree-org/iree,iree-org/iree,google/iree