Add documentation for gpuR

[jwokaty]

CRAN's gpuR is in suggests for RamEx Bioconductor/Contributions#3700.

gpuR must be compiled with support for OpenCL. On nebbiolo1, I was able to install it after apt install pocl-source pocl-opencl-icd. If RamEx is accepted, it may take some effort to install on Windows.

[qibebt-bioinfo]

@jwokaty Thank you very much for your valuable suggestions. I have already added the information on "How to install OpenCL" in the readme.md and vignettes/RamEx.Rmd files. Thank you again for your helpful advice.