Add README.md

Author: TApplencourt

backends/cuda/README.md

@@ -0,0 +1,7 @@
+# CUDA Backend
+
+## VDPAU Stub
+
+The CUDA SDK includes interop headers (`cudaVDPAU.h`, `cuda_vdpau_interop.h`) that reference VDPAU types (`VdpDevice`, `VdpGetProcAddress`, etc.) from `<vdpau/vdpau.h>`. This system header is not always available.
+
+A stub is provided in `include/vdpau/vdpau.h` that defines the necessary VDPAU types so that the CUDA tracer can compiled.