Docs: Compiling on Linux requires OpenCL

[Technologov]

Compiling on Ubuntu 18.04
apt-get install -y clang-6.0 ninja-build pkg-config protobuf-compiler libprotobuf-dev meson libopenblas-dev
is not enough; also needed is:
"ocl-icd-opencl-dev" (and this is on NVIDIA GPU card)

Problem is in docs.

Suggestion to add full line to docs:
https://github.com/LeelaChessZero/lc0

apt-get install -y clang-6.0 ninja-build pkg-config protobuf-compiler libprotobuf-dev meson libopenblas-dev ocl-icd-opencl-dev