update readme with note for OpenCL 2.0
update readme with note for OpenCL 2.0

tkelman/OpenCL.jl,yuyichao/OpenCL.jl,JuliaPackageMirrors/OpenCL.jl,tkelman/OpenCL.jl,JuliaPackageMirrors/OpenCL.jl,yuyichao/OpenCL.jl