maintainers/scripts/bootstrap-files: document the procedure of adding a new target

   who can help you test the updated architecture and once reviewed tag

  `@lovesegfault` to upload the tarballs.

## How to add bootstrap files for a new target

The procedure to add a new target is very similar to the update

procedure. The only difference is that you need to set up a new job to

build the `bootstrapFiles`. To do that you will need the following:

1. Add your new target to `lib/systems/examples.nix`

   This will populate `pkgsCross.$target` attribute set. If you are

   dealing with `bootstrapFiles` upload you probably already have it.

2. Add your new target to

   `pkgs/stdenv/linux/make-bootstrap-tools-cross.nix`. This will add a

   new hydra job to `nixpkgs:cross-trunk` jobset.

3. Wait for a hydra to build your bootstrap tarballs.

4. Add your new target to

   `maintainers/scripts/bootstrap-files/refresh-tarballs.bash` around

   `CROSS_TARGETS=()`.

5. Add your new target to `pkgs/stdenv/linux/default.nix` and follow

   standard bootstrap seed update procedure above.

## Bootstrap files job definitions

There are two types of bootstrap files: