Add mention to doc that Git LFS is needed to setup / build project

I have noticed that the following files are above github file size limit, and is thus requiring git lfs to checkout/pull:

Skipped checkout for "donut/nvrhi/thirdparty/Vulkan-Headers/include/vulkan/vulkan_structs.hpp", content not local. Use fetch to download.
Skipped checkout for "donut/nvrhi/thirdparty/Vulkan-Headers/registry/validusage.json", content not local. Use fetch to download.
Skipped checkout for "donut/thirdparty/imgui/examples/libs/glfw/lib-vc2010-32/glfw3.lib", content not local. Use fetch to download.
Skipped checkout for "donut/thirdparty/imgui/examples/libs/glfw/lib-vc2010-64/glfw3.lib", content not local. Use fetch to download.
Skipped checkout for "donut/thirdparty/lz4/tests/goldenSamples/skip.bin", content not local. Use fetch to download.
Skipped checkout for "donut/thirdparty/stb/tests/oversample/oversample.exe", content not local. Use fetch to download.
Skipped checkout for "media/glTF-Sample-Models/2.0/Box With Spaces/glTF/Box With Spaces.bin", content not local. Use fetch to download.
Skipped checkout for "media/glTF-Sample-Models/2.0/BrainStem/glTF-Draco/BrainStem0.bin", content not local. Use fetch to download.
Skipped checkout for "media/glTF-Sample-Models/2.0/BrainStem/glTF/BrainStem0.bin", content not local. Use fetch to download.
Skipped checkout for "media/glTF-Sample-Models/2.0/Sponza/glTF/Sponza.bin", content not local. Use fetch to download.
Checking out LFS objects: 100% (10/10), 31 MB | 0 B/s, done.

It would be nice if the README.md mentions that git LFS is needed as otherwise the project could not be built, as it would be missing some files. (Mainly the first errors would be related to vulkan structs headers missing). This should also reduce the confusion on checking out the example project.

Update README.md to explain that Git LFS is required to check out all files and build the project, including the missing Vulkan headers and example assets. Start by reading the existing setup and build instructions, then verify that the new note clearly warns newcomers about the prerequisite and checkout confusion.