docs: Add Vulkan to features.txt

docs: Add Vulkan to features.txt

To get the extension list:

$ git grep -hE "extension name=\"VK_KHR" src/vulkan/registry/vk.xml | \
  grep -v disabled | awk '{print $2}' | sed -E 's/(name=)?"//g' | sort

To find anv(il) and radv supported extensions:

$ git grep -hE "'VK_([A-Z]+)_[a-z]" src/intel/

$ git grep -hE "'VK_([A-Z]+)_[a-z]" src/amd/

v2:
 * Add radv to Vulkan 1.0 list (Bas)
 * 'started' => 'in progress'
 * Drop KHX and EXT extensions (Jason)

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>