Document required Vulkan extensions

[therontarigo]

Knowing ahead of time that the project uses VK_KHR_RAY_TRACING_PIPELINE and is not a fully software raytracing implementation could have saved some time.

Full list (from PBRVulkan/RayTracer/src/Vulkan/Device.cpp f19ca90)

VK_KHR_SWAPCHAIN
VK_KHR_SHADER_CLOCK
VK_KHR_PIPELINE_LIBRARY
VK_KHR_DEFERRED_HOST_OPERATIONS
VK_KHR_ACCELERATION_STRUCTURE
VK_KHR_RAY_TRACING_PIPELINE
VK_KHR_BUFFER_DEVICE_ADDRESS