[DOC] Add SYCL_INTEL_bf16_conversion status to README (#4428)

Author: AlexeySotkin

sycl/doc/extensions/README.md

@@ -44,6 +44,7 @@ DPC++ extensions status:
 | [Assert](Assert/SYCL_ONEAPI_ASSERT.asciidoc) | Proposal | |
 | [Matrix](Matrix/dpcpp-joint-matrix.asciidoc)                                                                        | Partially supported(AMX AOT)               | Not supported: dynamic-extent, wg and wi scopes, layouts other than packed|
 | [SYCL_INTEL_free_function_queries](FreeFunctionQueries/SYCL_INTEL_free_function_queries.asciidoc)                           | Supported (experimental)                  | |
+| [SYCL_INTEL_bf16_conversion](Bf16Conversion/SYCL_INTEL_bf16_conversion.asciidoc)                                            | Partially supported (Level Zero: GPU)     | Currently available only on Xe HP GPU. ext_intel_bf16_conversion aspect is not supported. |
 
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