Add documentation about CLBlast

[SlyEcho]

Installing, compiling and using CLBlast

Relevant issue: #1546

[arch-btw]

Thank you! Just a heads up for those on older AMD devices, I got a step further by adding:

ROC_ENABLE_PRE_VEGA=1

For example without ROC_ENABLE_PRE_VEGA=1 it outputs:

ggml_opencl: selected platform 'AMD Accelerated Parallel Processing' does not have any devices.

And with ROC_ENABLE_PRE_VEGA=1 it outputs:

ggml_opencl: selecting platform: 'AMD Accelerated Parallel Processing'
ggml_opencl: selecting device: 'gfx803'
ggml_opencl: device FP16 support: true

Granted, now I ran into this issue, but I think that's unrelated:

ggml_opencl: (queue = clCreateCommandQueue(context, device, CL_QUEUE_OUT_OF_ORDER_EXEC_MODE_ENABLE, &err), (err != CL_INVALID_QUEUE_PROPERTIES && err != CL_INVALID_VALUE ? err : (queue = clCreateCommandQueue(context, device, 0, &err), err) )) error -6 at ggml-opencl.cpp:490