README utility for `main` branches on the build repos on Hub

[sayakpaul]

After #287 is done, add a utility to generate a README for the main branch that points to the revisions because we have started recommending versions when downloading a kernel via get_kernel().

In doing so, it might so happen that the user gets treated with a blank page when opening a kernel repo (example https://huggingface.co/kernels-community/sgl-flash-attn3) even though it might have content in other revisions. This could be misleading.