Update README.md to add AMD GPU instructions

[zhangnju]

HI @GeeeekExplorer

Thanks for your great work on nano-vllm. I just tried on both AMD CDNA dataceter GPU and AMD RDNA3/4 desktop GPU, and it can work on both of them. so I listed the instructions of running it on AMD GPUs to share with the community.

Thanks

[alexhegit]

Better to readjust the markdown format to ensure proper line breaks.