readme: link to sha256sums file

[NicoWeio]

This is to emphasize that these do not need to be obtained from elsewhere.

[Technetium1]

I like this change, but the URL generated is not okay in rich diff, it goes to this invalid place:
https://github.com/ggerganov/llama.cpp/blob/link-checksums/SHA256SUMS
For this reason, probably should directly link to it:
https://github.com/ggerganov/llama.cpp/blob/master/SHA256SUMS

[NicoWeio]

I think this indeed only affects the preview (rich diff). If you use the link in https://github.com/NicoWeio/llama.cpp/blob/link-checksums/README.md, for example, it works just fine.
Sure, one could insert the full GitHub link, but this would prevent folks from opening the local checksum file right in their local editor, put a kind of hardlink to the base repo into forks, and possibly make the link go across branches (or break it). Apart from fixing the preview, I don't see any advantage. 🤔