maintainers/scripts/bootstrap-files: documentation and a script to update tarballs

This script attempts to document the exact procedure used to upload
bootstrap binaries used previously. I modeled it after most recent
https://github.com/NixOS/nixpkgs/pull/282517 upload.

There is one deviation from it to make it easier to handle mass updates
for https://github.com/NixOS/nixpkgs/issues/253713:

The binaries are expected to be stored in `stdenv/$target` (and not
something like `stdenv-linux/i686`.

The script handles both native and cross- linux targets. `darwin` will
need a bit more work to fin into this scheme, but it should be easy.

Example run to generate `i686-linux` update:

    $ maintainers/scripts/bootstrap-files/refresh-tarballs.bash --commit --targets=i686-unknown-linux-gnu

0 files changed, 0 insertions, 0 deletions