[HIPIFY][doc] Readme update
[HIPIFY][doc] Readme update

Clang 7.0.0 release has a known bug https://bugs.llvm.org/show_bug.cgi?id=36384, thus 7.0.0 is unstable release for Linux and Windows.
Last stable and tested LLVM+Clang release is 6.0.1.

ROCm-Developer-Tools/HIP,ROCm-Developer-Tools/HIP,ROCm-Developer-Tools/HIP,GPUOpen-ProfessionalCompute-Tools/HIP,ROCm-Developer-Tools/HIP,GPUOpen-ProfessionalCompute-Tools/HIP,GPUOpen-ProfessionalCompute-Tools/HIP,ROCm-Developer-Tools/HIP,GPUOpen-ProfessionalCompute-Tools/HIP,GPUOpen-ProfessionalCompute-Tools/HIP