doc: Add note on Vulkan-Headers repo 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/+/2144538 Reviewed-by: Ian Elliott <ianelliott@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Tobin Ehlis <tobine@google.com>