[SYCL][NATIVECPU] update docs for oneTBB integration

Author: uwedolinsky

sycl/doc/design/SYCLNativeCPU.md

@@ -62,6 +62,20 @@ in order to use a local checkout of the oneAPI Construction Kit. The CMake varia
 
 The SYCL Native CPU device needs to be selected at runtime by setting the environment variable `ONEAPI_DEVICE_SELECTOR=native_cpu:cpu`. 
 
+# oneTBB integration
+
+SYCL Native CPU can use oneTBB as an optional backend for task scheduling. oneTBB with SYCL Native CPU is enabled by setting `NATIVECPU_WITH_ONETBB=On` at configure time:
+
+```
+python3 buildbot/configure.py \
+  --native_cpu \
+  --cmake-opt=-DNATIVECPU_WITH_ONETBB=On
+```
+
+This will pull oneTBB into SYCL Native CPU via CMake `FetchContent` and DPC++ can be built as usual.
+
+By default SYCL Native CPU implements its own scheduler whose only dependency is standard C++.
+
 # Supported features and current limitations
 
 The SYCL Native CPU flow is still WIP, not optimized and several core SYCL features are currently unsupported. Currently `barriers` are supported only when the oneAPI Construction Kit integration is enabled, several math builtins are not supported and attempting to use those will most likely fail with an `undefined reference` error at link time. Examples of supported applications can be found in the [runtime tests](https://github.com/intel/llvm/blob/sycl/sycl/test/native_cpu).