Added a note about other Vulkan repos being dependent on Vulkan-Headers
repo. When the Headers repo roll fails, we need to pause it until after
LunarG has manually updated the dependent repos with new Vulkan-Headers
code.

Bug: angleproject:1944
Change-Id: I5ac2b061ff7e262595760645a843850b1235332f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2144538Reviewed-by: Ian Elliott <ianelliott@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Tobin Ehlis <tobine@google.com>