add some forgotted files

Author: Dmitry Razdoburdin

plugin/sycl/README.md

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+# SYCL-based Algorithm for Tree Construction
+This plugin adds support of SYCL programming model for tree construction and prediction algorithms to XGBoost.
+
+## Usage
+Specify the 'device' parameter as one of the following options to offload model training and inference on SYCL device.
+
+### Algorithms
+| device | Description |
+| --- | --- |
+sycl | use default sycl device  |
+sycl:gpu | use default sycl gpu  |
+sycl:cpu | use default sycl cpu  |
+sycl:gpu:N | use sycl gpu number N |
+sycl:cpu:N | use sycl cpu number N |
+
+Python example:
+```python
+param['device'] = 'sycl:gpu:0'
+```
+
+## Dependencies
+Building the plugin requires Data Parallel C++ Compiler (https://software.intel.com/content/www/us/en/develop/tools/oneapi/components/dpc-compiler.html)
+
+## Build
+From the command line on Linux starting from the xgboost directory:
+
+```bash
+$ mkdir build
+$ cd build
+$ EXPORT CXX=dpcpp && cmake .. -DPLUGIN_SYCL=ON
+$ make -j
+```