The Nixpkgs documentation on the linux kernel builders focused on
using and extending kernels that were already packaged, but never
mentioned that it's possible to also build a kernel almost "from
scratch".
The NixOS documentation went a bit deeper on manual linux kernel
configs, but that information wasn't particularly NixOS-specific.
This commit consolidates the information related to building the
kernel on Nixpkgs's documentation, while keeping any additional
NixOS-specific information on NixOS's documentation.
An additional README.md was created for contributor-facing
documentation.
While the word 'simply' is usually added to encourage readers, it often has the
opposite effect and may even appear condescending, especially when the reader
runs into trouble trying to apply the suggestions from the documentation. It is
almost always an improvement to simply drop the word from the sentence.
(there are more possible improvements like this, we can apply those in separate
PRs)
