chore(docs): reconcile submodule dep info

Remove explicit submodule version info and refer instead to the Git
submodules file. Also add some information for users about how to keep
the repo in sync.

Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
Change-Id: Iad89dc5f726bb64c65e91a505ac51cc73a905933
3 files changed