docs: Update docs with env vars
docs: Update docs with env vars

Added table of all environmental variables.  Also added note
about using LD_BIND_NOW usage for discovering problematic ICDs.

Change-Id: Iba4234b2226ab145491fc913f4361b098b21fd0b

KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers